#include<bits/stdc++.h>
using namespace std;
const int MAXN=200050;
int n,a[MAXN],frnt[MAXN],bck[MAXN],curr,cb=INT_MAX;
int main(){
cin>>n;
for (int i=1;i<=n;i++) cin>>a[i];
curr=a[1];
for (int i=2;i<=n;i++){
if (curr+1>a[i]+frnt[i-1]){
curr++;
frnt[i]=max(frnt[i-1],curr-a[i]);
}else{
curr=a[i]+frnt[i-1];
frnt[i]=frnt[i-1];
}
}
curr=a[n];
for (int i=n-1;i>=1;i--){
if (curr+1>a[i]+bck[i+1]){
curr++;
bck[i]=max(bck[i+1],curr-a[i]);
}else{
curr=a[i]+bck[i+1];
bck[i]=bck[i+1];
}
}
for (int i=1;i<=n;i++) cb=min(cb,max(frnt[i],bck[i]));
cout<<cb<<"\n";
}
