#include <iostream>
#include <fstream>
using namespace std;

ifstream fin("deque.in");
ofstream fout("deque.out");

const int MAX_LENGTH = 5000000;
const int MAX_VALUE = 10000000;

int nums[MAX_LENGTH + 1];

int main() {
    int numsLen, seqLen;
    fin >> numsLen >> seqLen;
    for (int i = 1; i <= numsLen; ++i) {
        fin >> nums[i];
    }
    long long sum = 0;
    for (int st = 1; st <= numsLen - seqLen + 1; ++st) {
        int minNumber = MAX_VALUE;
        for (int end = st; end <= st + seqLen - 1; ++end) {
            minNumber = min(nums[end], minNumber);
        }
        sum += minNumber;
    }
    fout << sum;
    return 0;
}