#include <cstdint>
#include <deque>
#include <fstream>
#include <tuple>
using namespace std;

int main() {
    ifstream fin("deque.in");
    ofstream fout("deque.out");
    
    int64_t n, k;
    deque<pair<int64_t, int64_t>> dq;
    
    fin >> n >> k;
    for (int64_t x, i = 0; i < k; ++i) {
        fin >> x;
        while (!dq.empty() && dq.back().second >= x)
            dq.pop_back();
        
        dq.push_back({i, x});
    }
    
    int64_t sum = dq.front().second;
    for (int64_t x, front = 0, back = k;
            back < n;
            ++front, ++back) {
        fin >> x;
        
        if (dq.front().first == front)
            dq.pop_front();
            
        while (!dq.empty() && dq.back().second >= x)
            dq.pop_back();
            
        dq.push_back({back, x});
        sum += dq.front().second;
    }
    
    fout << sum << endl;
    
    return 0;
}