#include <iostream>
#include <vector>
#include <cstdio>
#include <sstream>
#include <map>
#include <string>
#include <algorithm>
#include <queue>
#include <cmath>
#include <functional>
#include <set>
#include <ctime>
#include <random>
using namespace std;
template<typename T> istream& operator >> (istream& is, vector<T>& vec){for(T& val: vec) is >> val; return is;}
template<typename T> istream& operator , (istream& is, T& val){ return is >> val;}
template<typename T> ostream& operator << (ostream& os, const vector<T>& vec){for(int i=0; i<vec.size(); i++) os << vec[i] << (i==vec.size()-1?"\n":" ");return os;}
template<typename T> ostream& operator , (ostream& os, T& val){ return os << " " << val;}
template<typename T> ostream& operator >> (ostream& os, T& val){ return os << " " << val;}

using namespace std;

constexpr double EPS = 1e-7;

int main(){
	int n,l;
	cin >> n,l;
	vector<int> x(n, 0);
	for(int i=1; i<n; i++){
		cin >> x[i];
	}

	vector<double> g(n, 0);
	for(int i=n-1, sum=0; i>=0; i--){
		sum += x[i];
		g[i] = sum/1.0/(n-i);
	}

	vector<int> s(n, 0);
	for(int i=n-2; i>=0; i--){
		s[i] = 100 * (l - abs(x[i] - x[i+1]));
		if(s[i] < 100){
			cout << -1 << endl;
			return 0;
		}
	}

	int ans = 0;
	for(int i=n-2; i>=0; i--){
		if(( x[i] + EPS <= g[i+1]+0.5*l && g[i+1]+0.5*l <= x[i] + l - EPS ) && 
		   ( x[i+1] + EPS <= g[i+1]+0.5*l && g[i+1]+0.5*l <= x[i+1] + l - EPS ) ){
		}else{
			ans++;
		}
	}

	
	cout << ans << endl;
	return 0;
}