#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;
#define EPS 1e-6

int N, L;

int main(){
    ios::sync_with_stdio(0);
    cin.tie(0);
    cin >> N >> L;
    vi X(N), sum(N);
    rep(i, N-1)cin >> X[i + 1];
    for(int i = N - 2; i >= 0; --i)sum[i] = sum[i + 1] + X[i + 1];
    int ans = N - 1;
    rep(i, N - 1){
        double g = sum[i] / (double)(N - 1 - i);
        double al = X[i] - L*0.5;
        double ar = X[i] + L*0.5;
        double bl = X[i+1] - L*0.5;
        double br = X[i+1] + L*0.5;
        if(bl - ar > -EPS || al - br > -EPS){
            cout << -1 << endl;
            return 0;
        }
        if(al + EPS < g && g < ar - EPS && bl < g && g < br - EPS){
            ans--;
        }
    }
    cout << ans << endl;
}