#include<fstream>
#include<deque>
using namespace std;
ifstream fin("deque.in");
ofstream fout("deque.out");
const int NMAX = 5e6 + 5;

deque<int> Deque;

long long sum;
int Array[NMAX];
int ArraySZ, SeqSZ;

int main(){

    fin >> ArraySZ >> SeqSZ;

    int idx;
    for(idx = 1; idx < SeqSZ; ++idx){

        fin >> Array[idx];

        while(!Deque.empty() and Array[Deque.back()] > Array[idx])
            Deque.pop_back();
        Deque.push_back(idx);
    }

    for(idx = SeqSZ; idx <= ArraySZ; ++idx){

        fin >> Array[idx];

        while(!Deque.empty() and Array[Deque.back()] > Array[idx])
            Deque.pop_back();
        Deque.push_back(idx);

        sum += Array[Deque.front()];

        if(Deque.front() <= idx - SeqSZ + 1)
            Deque.pop_front();
    }
    fout << sum;
}