#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <string>
#include <cmath>
#include <climits>
#include <algorithm>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <cassert>
#include <vector>
#define all(x) x.begin() , x.end()
#define fi first
#define se second
#define pb push_back
#define umax( x , y ) x = max( x , (y) )
#define umin( x , y ) x = min( x , (y) )
#define For( i , a ) for(int i=1;i<=a;i++)
#define ort (b+s)/2
#define y2 asrwjaelkf
#define y1 asseopirwjaelkf
#define set multiset

using namespace std;

inline int read() {
    int res = 0 ;int neg ;
    while(true){char ch = getchar();if(ch>='0' && ch<='9' || ch=='-'){if(ch=='-') neg = -1;else neg = 1 , res = ch-'0';break;} }
    while(true){char ch = getchar();if(ch>='0' && ch<='9') res*=10 , res+=ch-'0';else break;}
    return res*neg;
}


typedef long long Lint;
typedef double db;
typedef pair<int,int> ii;
typedef pair<db,db> dd;
typedef pair<ii,int> iii;
typedef pair<ii,ii> i4;

const int maxn = 400320;
const int maxm = 1000020;
const int MOd = 1e9 + 7;

int a, b, ar[maxn], mini[maxn];
Lint dn[maxn];

int main() {


   // freopen("asd.in","r",stdin);
    //freopen("out.txt","w",stdout);


    scanf("%d %d",&a,&b);

    for(int i=1;i<=a;i++)
        scanf("%d",&ar[i]);
    mini[a] = ar[a];
    for(int i=a-1;i>=1;i--) mini[i] = min( mini[i+1], ar[i] );

    Lint ans = 1e18;
    for(int i=1;i<=b+1;i++) {
        Lint h = 0;
        for(int j=i;j<=a;j+=b+b+1) {
            h += ar[j];
            if( j+b >= a ) umin( ans, h );
        }
    }
    cout << ans << endl;
    return 0;

    for(int i=a;i>=1;i--) {
        int k = i;
        if( k > a ) k = a;
        dn[i] = 1e18;
        for(int j=-b;j<=b && i+j <= a;j++,k++) {
            if( i+j < 1 ) continue;
            if( k > a ) k = a;
            umin( dn[i], ar[i+j] + dn[k+1] );
        }
    }
    cout << dn[1] << endl;
    return 0;
}