#include <iostream>
#include <cstdio>

using namespace std;

typedef long long LL;

const int N = 100010;

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

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

    return 0;
}