#include<bits/stdc++.h>
using namespace std;
int n,m,a[3005],cnt[3005],minn=105;

int main()
{
    cin>>n>>m;
    for(int i=0;i<n;++i)
        cin>>a[i];
    for(int i=0;i<n-m+1;++i)
    {
        cnt[i]=a[i]+a[i+1]+a[i+2];
        if(cnt[i]<minn) minn=cnt[i];
    }
    cout<<minn;
    return 0;
}