#include <bits/stdc++.h>
using namespace std;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
long long n;cin >> n;
vector<long long> A(n);
for(int i = 0;i<n;i++){
cin >> A[i];
}
//sequence has to be increasing then decreasing
vector<long long> L(n);
vector<long long> R(n);
for(int i = 1;i<n;i++){
L[i] = L[i-1] + max(0LL,L[i-1]-L[i] + 1);
}
for(int i = n-2;i>=0;i--){
R[i] = R[i+1] + max(0LL,R[i+1] - R[i] + 1);
}
long long ans = LLONG_MAX;
for(int i = 0;i<n;i++){
ans = min(ans,max(R[i],L[i]));
}
cout << ans;
}
