#include<fstream>
#include<deque>
#include<algorithm>
using namespace std;

ifstream cin("deque.in");
ofstream cout("deque.out");

long long n, S=0, k, mini;
deque <long long> d;
int main(){
	cin>>n>>k;	
	long long x;
	
	for(int i=0; i<n; ++i){
		cin>>x; d.push_back(x);
		if(d.size()==k){
			mini=10000100;
				for(int j=1; j<k; j++){
					mini = min(min(d.at(j-1),d.at(j)),mini);
			}
			S+=mini;
			d.pop_front();
		}
	}
	cout<<S;
	
	return 0;
}