#include <bits/stdc++.h>
using namespace std;

int N;
int A[1000000000];

int main()
{
    cin >> N;

    for (int i = 0; i < N; ++i) cin >> A[i];


    int sum = 0;
    int j = 0;
    for (int i = 1; i <= N-1; i++) {
        while (true) {
            if (A[i-1] - (A[i] + j) <= 0) {
                sum += j;
                break;
            }
            else j++;
        }
    }
    cout << sum << endl;
}

