#include <bits/stdc++.h>
namespace{
using namespace std;
using ll = long long;
using ld = long double;
using ull = unsigned long long;
using uint = unsigned;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pdd = pair<ld, ld>;
using tuplis = array<ll, 3>;
template<class T> using pq = priority_queue<T, vector<T>, greater<T>>;
const ll LINF=0x1fffffffffffffff;
const ll MINF=0x7fffffffffff;
const int INF=0x3fffffff;
const int MOD=1000000007;
const int MODD=998244353;
const ld DINF=INFINITY;
const ld EPS=1e-9;
const ld PI=3.14159265358979323846;
const ll dx[] = {0, 1, 0, -1, 1, -1, 1, -1};
const ll dy[] = {1, 0, -1, 0, 1, 1, -1, -1};
#define overload5(a,b,c,d,e,name,...) name
#define overload4(a,b,c,d,name,...) name
#define overload3(a,b,c,name,...) name
#define rep1(n) rep2(_,n)
#define rep2(i,n) for(ll i=0;i<n;++i)
#define rep3(i,a,b) for(ll i=a;i<b;++i)
#define rep4(i,a,b,c) for(ll i=a;i<b;i+=c)
#define rep(...) overload4(__VA_ARGS__,rep4,rep3,rep2,rep1)(__VA_ARGS__)
#define rrep1(n) for(ll i=n;i--;)
#define rrep2(i,n) for(ll i=n;i--;)
#define rrep3(i,a,b) for(ll i=b;i-->(a);)
#define rrep4(i,a,b,c) for(ll i=(a)+((b)-(a)-1)/(c)*(c);i>=(a);i-=c)
#define rrep(...) overload4(__VA_ARGS__,rrep4,rrep3,rrep2,rrep1)(__VA_ARGS__)
#define each1(i,a) for(auto&&i:a)
#define each2(x,y,a) for(auto&&[x,y]:a)
#define each3(x,y,z,a) for(auto&&[x,y,z]:a)
#define each4(w,x,y,z,a) for(auto&&[w,x,y,z]:a)
#define each(...) overload5(__VA_ARGS__,each4,each3,each2,each1)(__VA_ARGS__)
#define all1(i) begin(i),end(i)
#define all2(i,a) begin(i),begin(i)+a
#define all3(i,a,b) begin(i)+a,begin(i)+b
#define all(...) overload3(__VA_ARGS__,all3,all2,all1)(__VA_ARGS__)
#define rall1(i) rbegin(i),rend(i)
#define rall2(i,a) rbegin(i),rbegin(i)+a
#define rall3(i,a,b) rbegin(i)+a,rbegin(i)+b
#define rall(...) overload3(__VA_ARGS__,rall3,rall2,rall1)(__VA_ARGS__)
#define sum(...) accumulate(all(__VA_ARGS__),0LL)
#define dsum(...) accumulate(all(__VA_ARGS__),0.0L)
#define Msum(...) accumulate(all(__VA_ARGS__),mint{})
#define elif else if
#define INT(...) int __VA_ARGS__;in(__VA_ARGS__)
#define LL(...) ll __VA_ARGS__;in(__VA_ARGS__)
#define ULL(...) ull __VA_ARGS__;in(__VA_ARGS__)
#define STR(...) string __VA_ARGS__;in(__VA_ARGS__)
#define CHR(...) char __VA_ARGS__;in(__VA_ARGS__)
#define DBL(...) double __VA_ARGS__;in(__VA_ARGS__)
#define LD(...) ld __VA_ARGS__;in(__VA_ARGS__)
#define vec(type,name,...) vector<type>name(__VA_ARGS__)
#define VEC(type,name,size) vector<type>name(size);in(name)
#define vv(type,name,h,...) vector name(h,vector<type>(__VA_ARGS__))
#define VV(type,name,h,w) vector name(h,vector<type>(w));in(name)
#define vvv(type,name,h,w,...) vector name(h,vector(w,vector<type>(__VA_ARGS__)))
template<class T> ll sz(const T& a){ return size(a); }
template<class T, class U> ll count(const T& a, const U& b){ return count(all(a), b); }
template<class T, class F> ll count_if(const T& a, F b){ return count_if(all(a), b); }
template<class T, class F> void filter(T& a, F b){ a.erase(remove_if(all(a), not_fn(b)), a.end()); }
template<class T, class F = less<>> void sor(T& a, F b = F{}){ sort(all(a), b); }
template<class T> void rev(T& a){ reverse(all(a)); }
template<class T> void uniq(T& a){ sor(a); a.erase(unique(all(a)), end(a)); }
ll popcnt(ull a){ return __builtin_popcountll(a); }
ll intpow(ll a, ll b){ ll ans = 1; while(b){ if(b & 1) ans *= a; a *= a; b /= 2; } return ans; }
ll modpow(ll a, ll b, ll p){ ll ans = 1; while(b){ if(b & 1) (ans *= a) %= p; (a *= a) %= p; b /= 2; } return ans; }
template<class T> T div_floor(T a, T b) { return a / b - ((a ^ b) < 0 && a % b); }
template<class T> T div_ceil(T a, T b) { return a / b + ((a ^ b) > 0 && a % b); }
template<class T> bool chmin(T& a, const T& b){ return a > b ? a = b, 1 : 0; }
template<class T> bool chmax(T& a, const T& b){ return a < b ? a = b, 1 : 0; }
template<class T, class U> bool chmin(T& a, const U& b){ return a > b ? a = b, 1 : 0; }
template<class T, class U> bool chmax(T& a, const U& b){ return a < b ? a = b, 1 : 0; }
vector<ll> iota(ll n, ll begin = 0){ vector<ll> a(n); iota(a.begin(), a.end(), begin); return a; }
vector<pll> factor(ull x){ vector<pll> ans; for(ull i = 2; i * i <= x; i++) if(x % i == 0){ ans.push_back({i, 1}); while((x /= i) % i == 0) ans.back().second++; } if(x != 1) ans.push_back({x, 1}); return ans; }
vector<ll> divisor(ull x){ vector<ll> ans; for(ull i = 1; i * i <= x; i++) if(x % i == 0) ans.push_back(i); rrep(ans.size() - (ans.back() * ans.back() == x)) ans.push_back(x / ans[i]); return ans; }
template<class T> unordered_map<T, ll> press(vector<T> a){ uniq(a); unordered_map<T, ll> ans; rep(i, a.size()) ans[a[i]] = i; return ans; }
template<class T> auto run_press(const T& a){ vector<pair<decay_t<decltype(a[0])>, ll>> ans; each(x, a){ if(ans.empty() || ans.back().first != x) ans.emplace_back(x, 1); else ans.back().second++; } return ans; }
template<class... Ts> void in(Ts&... t);
[[maybe_unused]] void print(){}
template<class T, class... Ts> void print(const T& t, const Ts&... ts);
template<class... Ts> void out(const Ts&... ts){ print(ts...); cout << '\n'; }
namespace IO{
#define VOID(a) decltype(void(a))
struct S{ S(){ cin.tie(nullptr)->sync_with_stdio(0); fixed(cout).precision(12); } }S;
template<int I> struct P : P<I-1>{};
template<> struct P<0>{};
template<class T> void i(T& t){ i(t, P<3>{}); }
void i(vector<bool>::reference t, P<3>){ int a; i(a); t = a; }
template<class T> auto i(T& t, P<2>) -> VOID(cin >> t){ cin >> t; }
template<class T> auto i(T& t, P<1>) -> VOID(begin(t)){ for(auto&& x : t) i(x); }
template<class T, size_t... idx> void ituple(T& t, index_sequence<idx...>){ in(get<idx>(t)...); }
template<class T> auto i(T& t, P<0>) -> VOID(tuple_size<T>{}){ ituple(t, make_index_sequence<tuple_size<T>::value>{}); }
template<class T> void o(const T& t){ o(t, P<4>{}); }
template<size_t N> void o(const char (&t)[N], P<4>){ cout << t; }
template<class T, size_t N> void o(const T (&t)[N], P<3>){ o(t[0]); for(size_t i = 1; i < N; i++){ o(' '); o(t[i]); } }
template<class T> auto o(const T& t, P<2>) -> VOID(cout << t){ cout << t; }
template<class T> auto o(const T& t, P<1>) -> VOID(begin(t)){ bool first = 1; for(auto&& x : t) { if(first) first = 0; else o(' '); o(x); } }
template<class T, size_t... idx> void otuple(const T& t, index_sequence<idx...>){ print(get<idx>(t)...); }
template<class T> auto o(T& t, P<0>) -> VOID(tuple_size<T>{}){ otuple(t, make_index_sequence<tuple_size<T>::value>{}); }
#undef VOID
}
#define unpack(a) (void)(0 + ... + (a, 0))
template<class... Ts> void in(Ts&... t){ unpack(IO::i(t)); }
template<class T, class... Ts> void print(const T& t, const Ts&... ts){ IO::o(t); unpack(IO::o((cout << ' ', ts))); }
#undef unpack
#ifdef DEBUG
ll __lg(ull x){ return 63 - __builtin_clzll(x); }
#define debug(...) { print(#__VA_ARGS__); print(":"); out(__VA_ARGS__); }
#else
#define debug(...) void(0)
#endif
#define YESNO(yes,no) void yes(bool i = 1){ out(i?#yes:#no); } void no(){ out(#no); }
YESNO(first, second)
YESNO(First, Second)
YESNO(Yes, No)
YESNO(YES, NO)
YESNO(possible, impossible)
YESNO(Possible, Impossible)
YESNO(POSSIBLE, IMPOSSIBLE)



void solve(){
    LL(n);
    VEC(ll,a,n);
    auto check=[=](const ll T)->bool{
        vector<pll>give; // [l, r) この区間もらう
        ll take=0,TL=T; // 何時までにいくつ送る
        each(x,a){
            if(take>TL)return 0;
            if(give.empty()){
                if(x+take<=TL){
                    take=0;
                    give.emplace_back(x,TL-take);
                    give.emplace_back(TL,T);
                }else{
                    TL-=take;
                    take=x-TL;
                }
            }else{
                const ll time=[&](ll x){
                    ll prev=0;
                    each(l,r,give){
                        {
                            const ll add=l-prev;
                            if(x<=add)return prev+x;
                            prev=l;
                            x-=add;
                        }
                        {
                            const ll add=(r-prev)*2;
                            if(x<=add)return prev+(x+1)/2;
                            prev=r;
                            x-=add;
                        }
                    }
                    return prev+x;
                }(x);
                give.clear();
                
                if(time<=T){
                    take=0;
                    give.emplace_back(time,T);
                }else{
                    take=time-T;
                    TL=T;
                }
            }
        }
        return take==0;
    };
    ll ok=ranges::max(a),ng=-1;
    while(ok-ng>1){
        const ll mid=(ok+ng)/2;
        (check(mid)?ok:ng)=mid;
    }
    out(ok);
}
}
int main(){
    ll t = 1;
//    in(t); // マルチテストケースかどうか確認！
    rep(i,t){
        solve();
    }
}