#include <iostream>
#include <cstdio>

using namespace std;

typedef long long LL;

const int N = 100010;

int n, len, x[N], cnt, l[N], r[N];
LL num, denom, ans;

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

    scanf("%d%d", &n, &len);
    l[1] = 0, r[2] = len - 1;
    for (int i = 2; i <= n; ++i) {
        scanf("%d", &x[i]);
        l[i] = x[i], r[i] = x[i] + len - 1;
    }

    ans = n - 1;
    for (int i = n; i >= 1; --i) {
        if (i <= n - 1) {
            if (max(l[i], l[i + 1]) * denom * 2 - denom < num * 2 && num * 2 < min(r[i], r[i + 1]) * denom * 2 + denom) {
                --ans;
            } else if (r[i] < l[i + 1] || l[i] > r[i + 1]) {
                puts("-1");
                return 0;
            }
        }
        denom += 2;
        num += l[i] + r[i];
    }

    printf("%lld\n", ans);

    return 0;
}