#include <iostream>

using namespace std;

int main(){
    int n;
    cin >> n;
    int niz[n];
    for (int i = 0; i < n; ++i){
        cin >> niz[i];
    }
    int rez = 0;
    for (int i = n - 2; i >= 0; --i){
        if (niz[i] >= niz[i + 1]){
           rez += niz[i] - niz[i + 1] + 1;
           niz[i] = niz[i + 1] - 1;
        }
    }
    cout << rez << endl;
    return 0;
}