#include <bits/stdc++.h>

using namespace std;
ifstream fin("deque.in");
ofstream fout("deque.out");

struct poz{
    int v,in;
};
deque<poz> q;
long long n,s,k,a;
int main()
{
    fin>>n>>k;

    for(int i=1;i<=k;++i)
    {
        fin>>a;
        while(q.empty()==false && a<q.back().v)
            q.pop_back();
        q.push_back({a,i});
    }

    s=q.front().v;
    for(int i=k+1;i<=n;++i)
    {
        fin>>a;
        while(q.empty()==false && q.front().in<i-k+1)
            q.pop_front();
        while(q.empty()==false && a<q.back().v)
            q.pop_back();
        q.push_back({a,i});

        s=s+q.front().v;
    }

    fout<<s;
    return 0;
}