#pragma GCC optimize ("03,unroll-loops")
#include <bits/stdc++.h>
using namespace std;
ifstream fin("deque.in");
ofstream fout("deque.out");
struct nr
{
    int ind;
    int val;
};
int main()
{
    deque<nr>deq;
    int n,k;
    fin>>n>>k;
    int i;
    int x;
    for(i=1;i<k;++i)
    {
        fin>>x;
        while(!deq.empty()&&deq.back().val>x)
            deq.pop_back();
        deq.push_back({i,x});
    }
    long long s=0;
    for(i=k;i<=n;++i)
    {
        fin>>x;
        if(deq.front().ind==i-k)
            deq.pop_front();
        while(!deq.empty()&&deq.back().val>x)
            deq.pop_back();
        deq.push_back({i,x});
        s+=deq.front().val;
    }
    fout<<s;
    return 0;
}