#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    int n, k;
    ll st = 0;
    cin >> n >> k;
    vector<ll> a(n);
    vector<ll> s(n - k + 1);
    vector<ll> pf(n - k + 1);
    vector<ll> sf(n - k + 1);
    for(int i = 0; i < n; ++i){
        cin >> a[i];
    }
    for(int i = 0; i < k; ++i){
        st += a[i];
    }
    s[0] = st;
    for(int i = 1; i < n - k + 1; ++i){
        s[i] = s[i - 1] + a[i + k - 1] - a[i - 1];
    }
    pf[0] = s[0];
    sf.back() = s.back();
    cout << "XD";
    for(int i = 1; i < n; ++i){
        pf[i] = max(s[i], pf[i - 1]);
        sf[i] = max(s[i], sf[n - i]);
    }
    ll ans = 1e18;
    for(int i = 0; i < n - k + 1; ++i){
        if(i - k < 0)
            ans = min(ans, sf[i + k]);
        else if(i + k > n - 1)
            ans = min(ans, pf[i - k]);
        else
            ans = min(ans, max(sf[i + k], pf[i - k]));
    }
    cout << ans << "\n";
}