#include <bits/stdc++.h>
using namespace std;
#define rep(i, a) for (int i = 0; i < (a); i++)
#define rep2(i, a, b) for (int i = (a); i < (b); i++)
#define repr(i, a) for (int i = (a) - 1; i >= 0; i--)
#define repr2(i, a, b) for (int i = (b) - 1; i >= (a); i--)
template<class T1, class T2> bool chmin(T1 &a, T2 b) { return b < a && (a = b, true); }
template<class T1, class T2> bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
typedef long long ll;

int main() {
	ll n, l;
	cin >> n >> l;
	vector<ll> x(n);
	rep2(i, 1, n) scanf("%lld", &x[i]);
	reverse(x.begin(), x.end());

	vector<ll> g(n);
	rep(i, n - 1) g[i + 1] = g[i] + (x[i] + x[i] + l);

	int ans = 0;

	rep2(i, 1, n) {
		ll L = x[i];
		ll R = x[i] + l;
		ll Lp = x[i - 1];
		ll Rp = x[i - 1] + l;
		if (R <= Lp || Rp <= L) {
			cout << -1 << endl;
			return 0;
		}

		bool ok = true;
		ok &= L * 2 * i < g[i] && g[i] < R * 2 * i;
		ok &= Lp * 2 * i < g[i] && g[i] < Rp * 2 * i;

		if (!ok) ans++;
	}

	cout << ans << endl;

	return 0;
}