func_code_string: #include <bits/stdc++.h>   using namespace std;   int n, m[100005], a[100005];   int main() {     scanf( %d , &n);     for (int i = 1; i <= n; ++i) {       scanf( %d , &a[i]);       m[i] = a[i] + 1;     }     long long ans = 0;     for (int i = n - 1; i >= 1; --i) m[i] = max(m[i], m[i + 1] - 1);     for (int i = 1; i <= n; ++i) m[i] = max(m[i], m[i - 1]);     for (int i = 1; i <= n; ++i) ans += (m[i] - a[i] - 1);     cout << ans << endl;     return 0;   }   