#include <cstdio>
#include <queue>
#define MIN 10000001
using namespace std;
deque <int> d;
int n,m,x,i,minim=MIN,j;
long long s=0;
int main()
{
    freopen("deque.in","r",stdin);
    freopen("deque.out","w",stdout);
    scanf("%d %d",&n,&m);
    for(i=1;i<=m;++i)
     {
         scanf("%d ",&x);
         d.push_back(x);
         if (x<minim) minim=x;
     }
     s=minim;
     for(i=m+1;i<=n;++i)
     {
         minim=MIN;
         scanf("%d",&x);
         d.pop_front();
         d.push_back(x);
          for(j=0;j<=m-1;++j)
            if (d[j]<minim) minim=d[j];
            s=s+minim;
     }
     printf("%d",s);
    return 0;
}