#include <iostream>

using namespace std;

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