#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <numeric>
#include <functional>
#include <cmath>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <sstream>
#include <string>
#define repd(i,a,b) for (int i=(a);i<(b);i++)
#define rep(i,n) repd(i,0,n)
#define var auto
#define mod 1000000007
#define inf 2147483647
#define mp make_pair
#define pb push_back
typedef long long ll;
using namespace std;

template <typename T>
inline void output(T a, int p) {
    if(p){
        cout << fixed << setprecision(p)  << a << "\n";
    }
    else{
        cout << a << "\n";
    }
}
// end of template

int main() {
    cin.tie(0);
    ios::sync_with_stdio(0);
    // source code
    
    int N, L;
    cin >> N >> L;
    L *= 2;
    if (N == 1) {
        output(0, 0);
        return 0;
    }
    vector<int> X(N, 0);
    repd(i, 1, N){
        cin >> X[i];
        X[i] *= 2;
    }
    
    reverse(X.begin(), X.end());
    
    int ret = 0;
    int sum = X[0];
    
    repd(i, 1, N){
        if (abs(X[i] - X[i - 1]) >= L) {
            output(-1, 0);
            return 0;
        }
        if (!(sum < i * (X[i - 1] + L / 2) && sum > i * (X[i - 1] - L / 2) && sum < i * (X[i] + L / 2) && sum > i * (X[i] - L / 2))) {
            ret++;
        }
        sum += X[i];
    }
    
    output(ret, 0);
    
    return 0;
}