#include<cstdio>
#include<vector>
#include<map>
using namespace std;
int main()
{
multimap<int, int> M;
int n, k, sum=0;
scanf("%d %d", &n, &k);
vector<int> arr(n);
for(int i=0; i<n; i++)
scanf("%d", &arr[i]);
for(int i=0; i<k; i++)
M.insert({arr[i], 0});
for(int i=0; i+k<n; i++)
{
auto it=M.begin()+(auto)k/2;
sum+=it->first;
M.erase(M.find(arr[i]));
M.insert({arr[i+k], 0});
}
printf("%d", sum);
}
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