#include <iostream>
#include <stdio.h>
#include <math.h>

using namespace std;

FILE *f=fopen("deque.in","r");
FILE *g=fopen("deque.out","w");

int i,v[5000000],m,n,k,s;

int main()
{
    fscanf(f,"%d%d",&n,&k);
    int m=10000000;
    for(i=1;i<=n;i++)
     fscanf(f,"%d",&v[i]);
    for(i=1;i<=k;i++)
     m=min(m,v[i]);
    for(i=k+1;i<=n;i++)
     {
         s=s+m;
         m=min(v[i],v[i-1]);
         m=min(m,v[i-2]);
     }
    fprintf(g,"%d",s+1);
    fclose(g);
    return 0;
}