#include <bits/stdc++.h>   using namespace std;   int main() {     ios::sync_with_stdio(false);     cin.tie(0);     int n;     cin >> n;     vector<long long> m(n);     for (int i = 0; i < n; i++) cin >> m[i];     long long sum = 0;     for (int i = 0; i < n; i++) sum += m[i];     sum += n;     vector<long long> a(n + 1);     for (int i = n - 1; i >= 0; i--) a[i] = max(a[i + 1] - 1, m[i] + 1);     for (int i = 1; i < n; i++) a[i] = max(a[i], a[i - 1]);     for (int i = 0; i < n; i++) sum -= a[i];     cout << -sum << endl;     return 0;   }   