#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>
using namespace std;
typedef long long ll;
const int maxn = 1e5 + 100;
int n, l;
int a[maxn];
ll pref[maxn];

ll query(int i, int j) {
    if(i == 0) {
        return pref[j];
    }
    return pref[j] - pref[i - 1];
}

bool check(ll frogs) {
    for(int i = 0; i < n; i++) {
        if(i + l - 1 >= n) {
            return true;
        }
        if(query(i, i + l - 1) < frogs) {
            return false;
        }

    }
    return true;
}
int main() {
    ios_base::sync_with_stdio(false);
    cin >> n >> l;

    n--;
    ll sum = 0;
    for(int i = 0; i < n; i++) {
        cin >> a[i];

        sum += a[i];
        pref[i] = sum;
    }

    ll L = 0, R = 2e9;
    ll res= 0 ;
    while(L <= R) {
        ll mid = (L + R) / 2;

        if(check(mid)) {
            res = max(res, mid);
            L = mid + 1;
        }
        else {
            R = mid - 1;
        }
    }
    cout << res << endl;
    return 0;
}