#include <cstdio>

const int MAXN = 200;

int n, p[MAXN];

int main() {
	scanf("%d", &n);
	
	for(int i = 0; i < n; ++i)
		scanf("%d", p + i);
	
	int t = 0;
	for(int i = n - 2; i >= 0; --i)
		if(p[i] >= p[i + 1]) {
			int c = p[i] - p[i + 1] + 1;
			
			t += c;
			p[i] -= c;
		}
	
	printf("%d\n", t);
	
	return 0;
}