#include <bits/stdc++.h>

using namespace std;

int main()
{
    vector<int> v;
    int n, k;
    int64_t suma=0;

    cin>>n>>k;
    for(int i=1;i<=n;++i)
    {
        int x;
        cin>>x;
        v.push_back(x);
    }

     for(int i=k-1;i<=n;++i)
     {
        if(v[i-1] <= v[i] && v[i-1] <= v[i-2])
            suma+=v[i-1];
        else
        {
            if(v[i-2]<=v[i] && v[i-2]<=v[i-1])
                suma+=v[i-2];
            else
                suma+=v[i];
        }
     }

     cout<<suma;
    return 0;
}