#include<bits/stdc++.h>
using namespace std;
using ll=long long;
const int N=2e5+5;
int n,a[N],b[N],c[N],t[N];
bool chk(int x) {
    ll s=0;
    for(int i=1;i<=n;i++)b[i]=a[i],s+=x-b[i],t[i]=c[i]=0;
    if(s<0)return 0;
    ll now=0;
    for(int i=1;i<=n;i++) {
        if(b[i]>b[i-1]) {
            int v=min(t[i],(b[i]-b[i-1]+1)/2);
            b[i]=b[i-1]+v+max(0,b[i]-b[i-1]-2*v);
        }
        auto ck=[&](int y)->bool {
            if(i==1)return 1;
            if(b[i-1]<y)return b[i-1]<=c[i-1];
            int v=b[i-1]-y;
            return y+(v+1)/2<=c[i-1];
        };
        int l=0,r=x,mid,s=-1;
        while(l<=r)mid=l+r>>1,ck(mid)?l=mid+1,s=mid:r=mid-1;
        if(s==-1)return 0; c[i]=s;
        if(b[i]<c[i])t[i+1]+=c[i]-b[i];
    }return c[n]>=b[n];
}
int main() {
    ios::sync_with_stdio(0),cin.tie(0);
    cin>>n;
    for(int i=1;i<=n;i++)cin>>a[i];
    int l=0,r=1e9,mid,ans;
    while(l<=r)mid=l+r>>1,chk(mid)?r=mid-1,ans=mid:l=mid+1;
    cout<<ans<<'\n';
    return 0;
}