#include <fstream>
#include <deque>
using namespace std;

ifstream cin("deque.in");
ofstream cout("deque.out");

int v[5000005];

deque <int> dq;

int main()
{
    int n, k, a;
    double mn=0;
    cin>>n>>k;
    for(int i=1; i<=n; i++)
    {
        cin>>v[i];
        if(i<=k)
        {
            while(!dq.empty() && v[dq.back()]>v[i])
            {
                dq.pop_back();
            }
            dq.push_back(i);
        }
    }
    mn=v[dq.front()];
    for(int i=k+1; i<=n; i++)
    {
        while(dq.front()<=i-k)
        {
            dq.pop_front();
        }
        while(!dq.empty() && v[dq.back()]>v[i])
        {
            dq.pop_back();
        }
        dq.push_back(i);
        mn+=v[dq.front()];
    }
    cout<<mn;


    return 0;
}