include <bits/stdc++.h>

using namespace std;

define io iosbase::syncwith_stdio(0);cin.tie(0);

define int long long

signed main() { io

int n, k, a[100005];
cin >> n >> k;
n--;
for(int i=0; i<n; i++){
    cin >> a[i];
}
int l=0, r=0, sm = 0, ans = INT_MAX;
bool ol = false;
while(r<n){
    r++;
    if(ol) l++;
    sm += a[r-1];
    if(r-l == k){
        ans = min(ans, sm);
        sm -= a[l];
        ol = true;
    }
}
cout << ans;

}