#include <iostream>
using namespace std;

const int maxn = 100;

int n; // tasemeid
int p[maxn]; // tasemete punktid
int k = 0; // kustutatud punktid

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