#include <iostream>
#include <algorithm>
#include <iomanip>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <numeric>
#include <bitset>
#include <cmath>

static const int MOD = 1000000007;
using ll = long long;
using u32 = uint32_t;
using namespace std;

template<class T> constexpr T INF = ::numeric_limits<T>::max() / 32 * 15 + 208;

int main() {
    ll n, l;
    cin >> n >> l;
    l += l;
    vector<ll> v(n);
    for (int i = 1; i < n; ++i) {
        cin >> v[i];
        v[i] += v[i];
        if(abs(v[i]-v[i-1]) >= l){
            puts("-1");
            return 0;
        }
    }
    ll ans = n-1;
    ll s = 0;
    for (int i = n-1; i >= 1; --i) {
        s += v[i];
        if(max(v[i-1], v[i])*(n-i) < s+l/2*(n-i) && s+l/2*(n-i) < min(v[i-1]+l, v[i]+l)*(n-i)){
            ans--;
        }
    }
    cout << ans << "\n";
    return 0;
}