#include<deque>
#include<cstdio>
using namespace std;
deque<int> deq;
int a[5000001];
int n,i,k;
long long s;
int main()
{
    freopen("deque.in","r",stdin);
    freopen("deque.out","w",stdout);
    scanf("%d%d",&n,&k);
    for (i=1;i<=k;++i)
    {
        scanf("%d",&a[i]);
        while(!deq.empty() && a[deq.back()]>a[i])
                    deq.pop_back();
        deq.push_back(i);
        if (deq.front()==i-k) deq.pop_front();
        if(i>=k && !deq.empty()) s+=a[deq.front()];
    }
    printf("%lld\n",s);
    return 0;
}