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

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

	vector<int> xs(n-1);
	for (auto &x : xs)
	{
		scanf("%d", &x);
	}

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

	vector<double> gs(n - 1);
	gs[n - 2] = xs[n - 2];
	for (int i = n - 3; i >= 0; i--)
	{
		gs[i] = xs[i] + gs[i + 1];
	}

	for (int i = 0; i < n-1; i++)
	{
		double g = gs[i];
		if (n - 1 - i != 0)
			g /= n - 1 - i;
		if (xs[i] - l / 2.0 < g && g < xs[i] + l / 2.0)
		{
			if (i == 0)
			{
				if (-1 * l / 2.0 < g && g < l / 2.0)
					m--;
			}
			else if (xs[i - 1] - l / 2.0 < g && g < xs[i - 1] + l / 2.0)
				m--;
		}
	}

	cout << m << endl;
	return 0;
}