func_code_string: #include <bits/stdc++.h>   using namespace std;   const long long inf = 1e9 + 7;   const int maxn = 1e5 + 5;   int n, m;   int a[maxn];   int num[maxn];   int main() {     cin >> n;     long long sum1 = 0;     for (int i = 1; i <= n; i++) {       scanf( %d , &a[i]);       sum1 += a[i];     }     num[n] = a[n];     long long sum2 = 0;     for (int i = n - 1; i >= 1; i--) {       num[i] = max(a[i], num[i + 1] - 1);     }     for (int i = 1; i < n; i++) {       if (num[i] > num[i + 1]) {         num[i + 1] = num[i];       }     }     for (int i = 1; i <= n; i++) {       sum2 += num[i] + 1;     }     cout << sum2 - sum1 - n << endl;     return 0;   }   