#include <stdio.h>

int main()
{
	int i, N, A[100001];
	scanf("%d", &N);
	for (i = 1, A[0] = 0; i <= N; i++) scanf("%d", &(A[i]));
	
	int ans = 0;
	for (i = N; i >= 1; i--) if (A[i] > A[i-1] + 1) ans += A[i] - A[i-1] - 1;
	printf("%d\n", ans);
	fflush(stdout);
	return 0;
}