#include <iostream>
#include <cstdio>

using namespace std;

typedef pair<double, double> PDD;

const int N = 100010;
const double EPS = 1e-9;

int n, l, x[N], cnt;
PDD irons[N];

int main() {
    // freopen("skimp.in", "r", stdin);
    // freopen("skimp.out", "w", stdout);

    scanf("%d%d", &n, &l);
    irons[1] = { 1, l };
    for (int i = 2; i <= n; ++i) {
        scanf("%d", &x[i]);
        irons[i] = { x[i], irons[1].second + x[i] };
    }

    for (int i = 2; i <= n; ++i) {
        if (irons[i].first <= irons[i - 1].second) break;
        if (i == n) {
            puts("-1");
            return 0;
        }
    }

    double sum = (irons[n].first + irons[n].second) / 2;
    cnt = n - 1;
    for (int i = n - 1; i >= 1; --i) {
        double avg = 1.0 * sum / (n - i);
        if (avg - irons[i + 1].first > -EPS && irons[i + 1].second - avg > -EPS && avg - irons[i].first > -EPS && irons[i].second - avg > -EPS) {
            --cnt;
        }
        sum += (irons[i].first + irons[i].second) / 2;
    }

    printf("%d\n", cnt);

    return 0;
}