#include <iostream>
#include <vector>
#include <map>
#include <algorithm>
using namespace std;

#define REP(i,s,e) for (i = s; i <= e; i++)
#define rep(i,n) REP (i,0,(int)(n)-1)
#define RREP(i,s,e) for (i = s; i >= e; i--)
#define rrep(i,n) RREP (i,(int)(n)-1,0)
#define INF (int)1e8
#define MOD (int)(1e9+7)
#define ESP 1e-9

typedef long long ll;

int main(void) {
    int i, n, l;
    int x[100010];
    cin >> n >> l;
    x[0] = 0;
    REP (i,1,n-1) cin >> x[i];

    int ans = 0;
    double m = 0;
    RREP (i,n-1,1) {
        if (abs(x[i] - x[i-1]) >= l) {
            cout << -1 << endl;
            return 0;
        }
        m = (m * (n-i-1) + x[i]) / (n-i);
        if (m-ESP <= x[i]-l/2.0 || x[i]+l/2.0 <= m+ESP || m-ESP <= x[i-1]-l/2.0 || x[i-1]+l/2.0 <= m+ESP)
            ans++;
    }
    cout << ans << endl;
    return 0;
}