/* -*- coding: utf-8 -*-
 *
 * 343.cc: No.343 手抜き工事のプロ - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 100000;

/* typedef */

/* global variables */

int xs[MAX_N];

/* subroutines */

/* main */

int main() {
  int n, l;
  cin >> n >> l;

  xs[0] = 0;
  for (int i = 1; i < n; i++) cin >> xs[i];

  int cnt = 0;
  double lh = (double)l / 2, gcsum = 0.0;
  for (int i = n - 1; i > 0; i--) {
    gcsum += lh + xs[i];
    double gc = gcsum / (n - i);
    int ul0 = xs[i], ul1 = ul0 + l;
    int ll0 = xs[i - 1], ll1 = ll0 + l;
    //printf("gc[%d]=%lf\n", i, gc);
    //printf("  [%d-%d]-[%d-%d]\n", ul0, ul1, ll0, ll1);

    if (min(ul1, ll1) - max(ul0, ll0) < 1) {
      cnt = -1;
      break;
    }

    if (gc <= ul0 || gc >= ul1 || gc <= ll0 || gc >= ll1) cnt++;
  }

  printf("%d\n", cnt);
  return 0;
}