#include <iostream>
using namespace std;
const int N = 100010;
int a[N];
int n;
int main() {
cin >> n;
for (int i = 0; i < n; i ++) cin >> a[i];
int res = 0, max_score = a[0] + 0;
for (int j = 1; j < n; j ++) {
res = max(res, max_score + a[j] - j);
max_score = max(max_score, a[j] + j);
}
cout << res << endl;
return 0;
}