#include <bits/stdc++.h>
#define ll long long
#define ull unsigned long long
#define ld long double
#define len(v) (int)v.size()
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define pii pair<int, int>
#define vi vector<int>
#define vii vector<vector<int>>
#define vpii vector<pair<int, int>>
#define ull unsigned long long
//#define int long long
//#define ll ull
const int N = 1e6 + 1;
const int maxn = 5e6 + 10;
const int C = 20;
const int logn = 20;
const ll inf = 1e15;
const ll mod = 1e9 + 7;
const int M = 1e9;
const ull M2 = 998244353;
const ld eps = 1e-6;
using namespace std;

template<class T>
istream &operator>>(istream &in, vector<T> &a) {
    for (auto &i : a)
        in >> i;
    return in;
}

template<class T>
ostream &operator<<(ostream &out, vector<T> &a) {
    for (auto &i : a)
        out << i << ' ';
    return out;
}
void solve() {
    ll n, k; cin >> n >> k;
    vector<ll> a(n), b(n - k + 1, 0ull);
    cin >> a;
    ll su = 0;
    for (int i = 0; i < k; ++i)
        su += a[i];
    b[0] = su;
    for (int i = 1; i < n - k + 1; ++i) {
        b[i] = b[i - 1] + a[i + k - 1] - a[i - 1];
    }
    vector<ll> p(n - k + 1), s(n - k + 1);
    for (int i = k; i < n - k + 1; ++i)
        p[i] = max(p[i - 1], b[i - k]);
    for (int i = n - 2 * k; i >= 0; --i)
        s[i] = max(s[i + 1], b[i + k]);
    ll ans = inf;
    for (int i = 0; i < n - k + 1; ++i)
        ans = min(ans, max(p[i], s[i]));
    cout << ans;

}

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);

    int T = 1;
    //cin >> T;
    while (T--)
        solve();
}