#include <iostream>
#include <cstdio>

using namespace std;

typedef long long LL;
typedef pair<int, int> PII;

const int N = 100010;

int n, l, x[N], cnt;
LL num, denom, ans;
PII irons[N];

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

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

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

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

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

    return 0;
}