#define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES

#include <iostream>
#include <cstring>
#include <algorithm>
#include <limits.h>
#include <cmath>
#include <cstdio>
#include <vector>
#include <set>
#include <string>
#define REP(i,a,b) for(i=a;i<b;i++)
#define rep(i,n) REP(i,0,n)
using namespace std;

typedef long long ll;
typedef unsigned long long ull;

int test = 0;
/* ここからが本編 */

int main(void)
{
  int i,j,k,l;
  int n;
  int L;
  int x[100005];
  int rep = 0;
  cin >> n >> L;
  x[0] = 0;
  for(i=0;i+1<n;i++) {
    cin >> x[i+1];
    if((L-abs(x[i+1]-x[i])) * 100 < 100) {
      rep = -1;
      goto end;
    }
    if(L <= abs(x[i+1]-x[i])*2) rep++; 
  }
end:
  printf("%d\n",rep);
  return 0;
}