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

    for (int i = 2; i <= n; ++i) {
        if (l[i] < r[i - 1] || l[i - 1] > r[i]) {
            puts("-1");
            break;
        }
    }

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

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

    return 0;
}