#include<bits/stdc++.h>
#define ull unsigned long long
#define ui unsigned int
#define ld long double
#define ll long long
#define lll __int128
#define fi first
#define se second
#define e emplace
#define eb emplace_back
#define db double
#define ef emplace_front
#define pii pair<int,int>
#define pll pair<ll,ll>
#define vi vector<int>
#define vp vector<pii>
#define vt vector<tup>
#define all(x) x.begin(),x.end()
#define mp make_pair

#define FastI
#define FastO
#define int ll
bool ST;
static const ll MOD=1e9+7,Phi=998244352,inv2=499122177,Root=3,iRoot=332748118;
static const ll inf=1073741823,Inf=4294967296,INF=4557430888798830399;
static const ld eps=1e-9,pi=3.1415926535;
char in[1<<20],*p1=in,*p2=in;
char out[1<<20],*p3=out;
using namespace std;
struct tup
{
	int x,y;
	ll z;
	tup(int X=0,int Y=0,ll Z=0)
	{x=X,y=Y,z=Z;}
};
#ifdef FastI
#define getchar() (p1==p2&&(p2=(p1=in)+fread(in,1,1<<20,stdin),p1==p2)?EOF:*p1++)
#endif
#ifdef FastO
#define putchar(x) (p3-out==1<<20?fwrite(out,1,1<<20,stdout),p3=out,0:0,*p3++=x)
#define puts(x) write(x,'\n')
#endif
namespace FastIO
{
	template<typename T> inline void write(T x,char ch=' ')
	{
		if(is_same<char,T>::value)putchar(x);
		else
		{
			if(x<0)x=-x,putchar('-');
			static char st[25];
			int top=0;
			do st[top++]=x%10+'0',x/=10;while(x);
			while(top)putchar(st[--top]);
		}
		ch!='~'?putchar(ch):0;
	}
	inline void write(const char*x,char ch=' ')
	{
		for(int i=0;x[i]!='\0';++i)putchar(x[i]);
		ch!='~'?putchar(ch):0;
	}
	inline void read(char&s){do s=getchar();while(s=='\n'||s==' ');}
	inline void read(char s[])
	{
		int len=0;char st;
		do st=getchar();while(st=='\n'||st==' ');
		s[++len]=st,st=getchar();
		while(st!='\n'&&st!=' '&&st!='\r')s[++len]=st,st=getchar();
		s[++len]='\0';
	}
	template<typename T> inline void read(T &s)
	{
		char ch=getchar();s=0;
		while((ch>'9'||ch<'0')&&ch!='-')ch=getchar();
		bool tf=(ch=='-'&&(ch=getchar()));
		while(ch>='0'&&ch<='9')s=(s<<1)+(s<<3)+ch-'0',ch=getchar();
		s=tf?-s:s;
	}
	inline void edl(){putchar('\n');}
	template<typename T1,typename T2> inline void read(pair<T1,T2> &s){read(s.fi),read(s.se);}
	template<typename T,typename...Args> inline void write(T x,Args...args){write(x,'~'),write(args...);}
	template<typename T,typename...Args> inline void read(T&x,Args&...args){read(x),read(args...);}
	#ifdef FastO
	struct Writer{~Writer(){fwrite(out,1,p3-out,stdout);}}Writ;
	#endif
}
using namespace FastIO;
namespace MTool
{
	inline int Cadd(int a,int b){return (ll)a+b>=MOD?(ll)a+b-MOD:a+b;}
	inline int Cdel(int a,int b){return a-b<0?a-b+MOD:a-b;}
	inline int Cmul(int a,int b){return 1ll*a*b%MOD;}
	inline int sqr(int a){return 1ll*a*a%MOD;}
	inline void Madd(int&a,int b){a=((ll)a+b>=MOD?(ll)a+b-MOD:a+b);}
	inline void Mdel(int&a,int b){a=(a-b<0?a-b+MOD:a-b);}
	inline void Mmul(int&a,int b){a=1ll*a*b%MOD;}
	inline int Cmod(int x){return (x%MOD+MOD)%MOD;}
	inline void Mmod(int&x){x=(x%MOD+MOD)%MOD;}
	template<typename T> inline bool Mmax(T&a,T b){return a<b?a=b,1:0;}
	template<typename T> inline bool Mmin(T&a,T b){return a>b?a=b,1:0;}
	template<typename...Args> inline void Madd(int&a,int b,Args...args){Madd(a,b),Madd(a,args...);}
	template<typename...Args> inline void Mmul(int&a,int b,Args...args){Mmul(a,b),Mmul(a,args...);}
	template<typename...Args> inline void Mdel(int&a,int b,Args...args){Mdel(a,b),Mdel(a,args...);}
	template<typename...Args> inline int Cadd(int a,int b,Args...args){return Cadd(Cadd(a,b),args...);}
	template<typename...Args> inline int Cmul(int a,int b,Args...args){return Cmul(Cmul(a,b),args...);}
	template<typename...Args> inline int Cdel(int a,int b,Args...args){return Cdel(Cdel(a,b),args...);}
	template<typename...Args,typename T> inline bool Mmax(T&a,T b,Args...args){return Mmax(a,b)|Mmax(a,args...);}
	template<typename...Args,typename T> inline bool Mmin(T&a,T b,Args...args){return Mmin(a,b)|Mmin(a,args...);}
//	inline int power(int x,int y){int s=1;for(;y;y>>=1,Mmul(x,x))if(y&1)Mmul(s,x);return s;}
}
using namespace MTool;
namespace WrongAnswer_90
{
	int n;
	int a[200010];
	int typ[200010],L1[200010],R1[200010],L2[200010],R2[200010];
	inline bool chk(int x)
	{
		if(a[1]>x)typ[1]=0,R1[1]=x,L1[1]=2*x-a[1];
		else typ[1]=1,L2[1]=x+1,R2[1]=x,L1[1]=a[1]+1,R1[1]=x;
		for(int i=2;i<=n;++i)
		{
			if(typ[i-1])
			{
				if(R1[i-1]+R1[i-1]-L1[i-1]+1>=a[i])
				{
					typ[i]=1;
					L1[i]=(a[i]+L1[i-1])/2+1;
					R1[i]=x;
					L2[i]=x+1,R2[i]=x;
				}
				else if(R1[i-1]-L1[i-1]+1+L2[i-1]-1>=a[i])
				{
					typ[i]=1;
					L1[i]=a[i]-(R1[i-1]-L1[i-1]+1)+1;
					R1[i]=x;
					L2[i]=x+1,R2[i]=x;
				}
				else if(R1[i-1]-L1[i-1]+1+R2[i-1]-L2[i-1]+1+R2[i-1]>=a[i])
				{
					typ[i]=1;
					L1[i]=(a[i]-R1[i-1]+L1[i-1]+L2[i-1]-2+1)/2+1;
					R1[i]=x;
					L2[i]=x+1,R2[i]=x;
				}
				else
				{
					typ[i]=0;
					int ned=a[i]-x-(R1[i-1]-L1[i-1]+1+R2[i-1]-L2[i-1]+1);
					L1[i]=x-ned+1,R1[i]=x;
				}
			}
			else
			{
				if(a[i]>L1[i-1])typ[i]=0,L1[i]=2*a[i]-L1[i-1],R1[i]=L1[i-1]-1;
				else
				{
					typ[i]=1,L1[i]=a[i]+1,R1[i]=L1[i-1]-1;
					L2[i]=R1[i-1]+1,R2[i]=x;
				}
			}
			if(typ[i]==0&&L1[i]<0)return 0;
		}
		return typ[n];
	}
	inline void mian()
	{
		read(n);
		for(int i=1;i<=n;++i)read(a[i]);
		int L=0,R=INF,mid;
		while(L<R)
		{
			mid=(L+R)>>1;
			if(chk(mid))R=mid;
			else L=mid+1;
		}
		write(L);
	}
	inline void Mian()
	{
		int T=1;
//		read(T);
		while(T--)mian();
	}
}
bool ED;
signed main()
{
//	ios::sync_with_stdio(0);
//	freopen("1.in","r",stdin);
//	freopen("1.out","w",stdout);
	double st=clock();
	WrongAnswer_90::Mian();
	double ed=clock();
 	cerr<<endl;
 	cerr<<"Time: "<<ed-st<<" ms\n";
 	cerr<<"Memory: "<<abs(&ST-&ED)/1024.0/1024.0<<" MB\n";
	return 0;
}