#include <cstdio>

int main(void) {

	int N, A[1002];
	int ans = 0;

	scanf("%d", &N);
	for (int i = 0; i < N; ++i)
		scanf("%d", &A[i]);

	for (int i = N - 2; i >= 0; --i)
		if (A[i + 1] <= A[i]) {
			ans += A[i] - A[i + 1] + 1;
			A[i] = A[i + 1] - 1;
		}

	printf("%d\n", ans);

	return 0;
}