#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#define mid(L,R) ((L+R)/2)
#define lc(idx) (2*idx+1)
#define rc(idx) (2*idx+2)
using namespace std;
const int nax = 5 * 1e4;
typedef long long LL;

int main() {
    int N , K , i ;
    cin>>N>>K;
    vector < LL > A(N);
    vector < LL > dp(N);
    ++K;
    for(i=0;i<N;++i)cin>>A[i];
    for(i=N-1;i>=0;--i){
        if((i+K-1)>=N)
            dp[i] = 1e18;
        else
            dp[i] = A[i+K-1] + (i+K-1+K<N?dp[i+K-1+K]:0);
    }
    LL ans = 1e18;
    for(i=0;i<min(K,N);++i)ans = min(ans , A[i] + (i+K<N?dp[i+K]:0));

    cout<<(ans!=1e18?ans:-1);
    return 0;
}