#include<bits/stdc++.h>
#define int long long
#define inf 0x3f3f3f3f3f3f3f3f
#define maxn 1234567
#define eps 1e-7
#define mod 1000000007
#define Mod 998244353
#define f(i,a,b) for(int i=a;i<=b;i++)
#define r(i,a,b) for(int i=a;i>=b;i--) 
#define fx(i,a,b,x) for(int i=a;i<=b;i+=x)
#define rx(i,a,b,x) for(int i=a;i>=b;i-=x) 
using namespace std;
int T;
int n,ans;
double cen,L,a[maxn];
void solve(){
	scanf("%lld%lf",&n,&L);
	f(i,1,n-1){
		scanf("%lf",&a[i]);
		if(fabs(a[i]-a[i-1])-L>-eps){
			printf("-1\n");
			return;
		}
	}
	cen=a[n-1];
	r(i,n-2,0){
		if((cen-(a[i+1]-L/2)>eps)&&((a[i+1]+L/2)-cen>eps)&&(cen-(a[i]-L/2)>eps)&&((a[i]+L/2)-cen>eps))ans++;
		cen=(cen*(double)(n-i-1)+a[i])/(double)(n-i);
	}
	printf("%lld\n",n-1-ans);
}
signed main(){
	T=1;
//	scanf("%lld",&t);
	while(T--)solve();
	return 0;
}