#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("deque.in");
ofstream fout("deque.out");
int n,k,Front,Back,Deque[5000005],i,j,a[5000005],mini;
long long s;
int main()
{
    fin>>n>>k;
    Front=0; Back=1;
    for(i=1;i<k;i++)
        {
            fin>>a[i];
            Deque[++Front]=a[i];
        }
    for(;i<=n;i++)
    {
        fin>>a[i];
        if(a[i]<Deque[Front]) Deque[Front]=a[i];
        else Deque[++Front]=a[i];
        mini=Deque[Back];
        for(j=Back+1;j<=Front;j++) mini=min(mini,Deque[j]);
        s+=mini;
        if(Deque[Back]==a[i-2]) Back++;
    }
    fout<<s;
    return 0;
}