#include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <algorithm>
#include <set>
#include <sstream>
#include <utility>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <cctype>
#include <climits>
#include <iostream>
#include <iomanip>
#include <functional>
using namespace std;

typedef long long ll;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) for(int i=0;i<(n);++i)

#include <iostream>
using namespace std;

int main()
{
    ll n, l;
    cin >> n;
    cin >> l;
    vector<ll> xs(n);
    xs[0] = 0;
    for(int i = 1;i < n;i++)
    {
        cin >> xs[i];
    }
    if(n == 1)
    {
        cout << 0 << endl;
        return 0;
    }
    for(int i = 0; i < n-1;i++)
    {
        auto diff = xs[i+1]-xs[i];
        if(abs(diff) >= l)
        {
            cout << -1 << endl;
            return 0;
        }
    }
    vector<ll> sum(n);
    sum[n-1] = xs[n-1];
    for(ll i = n-2; i >= 0;i--)
    {
        sum[i] = sum[i+1] + xs[i];
    }
    int count = 0;
    for(ll i = n-2; i >= 0;i--)
    {
        auto moment = (sum[i+1] / (double)(n-(i+1))) + l / 2.0;
        //cout << "index " << i << "," << "moment " << moment << endl;
        //cout << "   " << moment -xs[i] << endl;
        //cout << "   " << moment -xs[i+1] << endl;
        //cout << endl;
        if(moment - xs[i] < l && moment - xs[i] >= 0 )
        {
            if(moment - xs[i+1] < l && moment -xs[i+1]>=0)
            {
                continue;
            }
        }
        //cout << "    count" << endl;
        count++;
    }
    cout << count << endl;
    return 0;
}