#include <bits/stdc++.h>
using namespace std;
int N;
int main() {
cin >> N;
vector<int> arr(N+1);
for (int i = 1; i <= N; i++) {
cin >> arr[i];
}
vector<int> prefixAdd(N+1), prefixVal(N+1);
for (int i = 1; i <= N; i++) {
prefixVal[i] = arr[i]+prefixAdd[i-1];
prefixAdd[i] = prefixAdd[i-1];
if (prefixVal[i] <= prefixVal[i-1]+1) {
prefixAdd[i] += (prefixVal[i-1]-prefixVal[i]+1);
prefixVal[i] = prefixVal[i-1]+1;
}
}
vector<int> suffixAdd(N+2), suffixVal(N+2);
for (int i = N; i >= 1; i--) {
suffixVal[i] = arr[i]+suffixAdd[i+1];
suffixAdd[i] = suffixAdd[i+1];
if (suffixVal[i] <= suffixVal[i+1]+1) {
suffixAdd[i] += (suffixVal[i+1]-suffixVal[i]+1);
suffixVal[i] = suffixVal[i+1]+1;
}
}
int ans = 1e9;
for (int i = 1; i <= N; i++) {
ans = min(ans, max(prefixAdd[i], suffixAdd[i]));
}
cout << ans << "\n";
}
