#include <bits/stdc++.h>
using namespace std;


int main()
{
	int n, l; scanf("%d", &n); scanf("%d", &l);
	if (n == 1) {
		printf("0\n"); return 0;
	}

	vector<int> xs(n);
	xs[0] = 0;
	for (int i = 1; i < n; i++) {
		scanf("%d", &xs[i]);
	}

	int m = 0;
	for (int i = 1; i < n; i++) {
		if (i == 0 && abs(xs[i]) >= l) {
			printf("-1\n"); return 0;
		}
		if (i != 0) {
			if (abs(xs[i - 1] - xs[i]) >= l) {
				printf("-1\n"); return 0;
			}
		}
		m++;
	}

	vector<double> gs(n);
	gs[n - 1] = xs[n - 1];
	for (int i = n - 2; i >= 0; i--) {
		gs[i] = xs[i] + gs[i + 1];
	}

	for (int i = 1; i < n; i++) {
		double g = gs[i] / (n - i);
		if (xs[i] - l / 2.0 < g && g < xs[i] + l / 2.0) {
			if (xs[i - 1] - l / 2.0 < g && g < xs[i - 1] + l / 2.0)
				m--;
		}
	}

	printf("%d\n", m); return 0;
}