#include <fstream>
using namespace std;
ifstream f("deque.in");
ofstream g("deque.out");
int N , k, v[5000005];
long long S;
int main()
{f >> N >> k;
 for(int i = 1 ; i <= N ; i++)
 f >> v[i];
 int pmin = 1 ;
 for(int i = 1 ; i <= N ; i++){
 if(v[i-1]>v[i] && v[pmin] > v[i]) pmin = i;
 if(i >= k){S = S + 1LL*min(v[pmin],v[i-k+1]); pmin = i;}}
 g << S;
 return 0;
}