#include <bits/stdc++.h>   long long mod = 1000000007;   using namespace std;   int n;   long long powmod(long long a, long long b) {     long long res = 1;     a %= mod;     for (; b; b >>= 1) {       if (b & 1) res = res * a % mod;       a = a * a % mod;     }     return res;   }   int a[100005];   int main() {     int i, j, fl, test, pos = 0, mx = 0;     scanf( %d , &n);     long long ans = 0;     for (i = 0; i < n; i++) {       scanf( %d , &a[i]);       if (a[i] > mx) {         mx = a[i];         pos = i;       }     }     for (i = pos; i < n; i++) {       ans = ans + (mx - a[i]);     }     int sofar[100005];     sofar[0] = 0;     for (i = 1; i < pos; i++) {       sofar[i] = max(sofar[i - 1], a[i]);     }     for (i = pos - 1; i >= 0; i--) {       mx--;       mx = max(mx, a[i]);       ans = ans + max((mx - a[i]), (sofar[i] - a[i]));     }     printf( %lld n , ans);     return 0;   }   