#include <iostream>
#include <cstdio>
#include <string>
#include <algorithm>
#include <vector>
#include <math.h>
#include <queue>

using namespace std;

int main() {
	int n; cin >> n;
	int p[101];
	for(int i = 0; i < n; ++i) {
		cin >> p[i];
	}

	int r = 0;
	for(int i = n - 2; i >= 0; --i) {
		if (p[i] >= p[i + 1]) {
			int d = p[i] - p[i + 1] + 1;
			r += d;
			p[i] -= d;
		}
	}

	cout << r << endl;

	//system("pause");
	return 0;
}