#include <bits/stdc++.h>
#define boAshraf ios_base::sync_with_stdio(false); cin.tie(NULL);
#define ll long long
#define sz(s) (int)(s).size()
#define all(s) (s).begin(),(s).end()
using namespace std;
void File();
void sol();
int main() {
    boAshraf
    File();
    int t = 1;
//    cin >> t;
    while (t--) {
        sol();
    }
    return 0;
}

void sol() {
    int n;
    cin>>n;
    vector<int>v(n);
    for(auto &it:v)cin>>it;
    sort(all(v));
    int m;
    cin>>m;
    auto valid=[&](int d)->bool{
        int cnt=0,last=-d;
        for(auto it:v){
            if(it-last>=d)cnt++,last=it;
        }
        return cnt>=m;
    };
    int ans=-1;
    int l=0,r=1e5,mid;
    while(l<=r){
        mid=l+(r-l)/2;
        if(valid(mid))ans=mid,l=mid+1;
        else r=mid-1;
    }
    cout<<ans;
}

void File() {
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
}