#include<bits/stdc++.h>
using namespace std;
#define ll long long
int n,k;
ll a,sum=0;
int main()
{
    std::ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
    cin>>n>>k;
    for(int i=1;i<=n;++i)
    {
        cin>>a;
        sum+=a;
    }
    sum-=n;
    cout<<sum/(n-k);
    return 0;
}