#include <iostream>
#include <utility>
#include <cstdio>
#include <deque>
using namespace std;
deque <pair <int,int> > v;
int n,k;
void rezolva();
int main()
{
    freopen("deque.in","r",stdin);
    freopen("deque.out","w",stdout);
    scanf("%d%d",&n,&k);
    rezolva();
    return 0;
}
void rezolva()
{
    long long s=0;
    int i,nr;
    for(i=1;i<k;i++)
    {
        scanf("%d",&nr);
        while(!v.empty()&& nr<v.back().second)
            v.pop_back();
        v.push_back(make_pair(i,nr));
    }
    for(i=1;i<=n;i++)
    {
        scanf("%d",&nr);
        while(v.empty()!=0 && nr<v.back().second)
            v.pop_back();
        v.push_back(make_pair(i,nr));
        if(v.front().first==i-k)
            v.pop_front();
        s+=v.front().second;
    }

    printf("%d",s);
}