#include<stdio.h>
int a[200005];
int main()
{
	int n;
	scanf("%d", &n);
	int i;
	for (i = 0; i < n; i++)
		scanf("%d", &a[i]);
	long long int ans = 0, f = 0;
	for (i = 1; i < n; i++)
	{
		if (a[i - 1] + 1 == a[i])
			f++;
		else
			f = 0;
		ans += f;
	}
	printf("%lld\n", ans);
	return 0;
}