#include<bits/stdc++.h>

using namespace std;
using ll = long long;


int main()
{
    ll n, k; cin >> n >> k;
    vector<ll> a(n);
    for(ll i = 0; i < n; i++)cin >> a[i];
    k--;
    if(!a[k]) {
        cout << 0 << endl;
        return 0;
    }

    ll sr = 0, sl = 0;
    ll ind = k+1;
    while(ind < n - 1 && a[ind] >= 2) {
        sr += a[ind];
        ind++;
    }
    sr += a[ind];
    ind = k-1;

    while(ind > 0 && a[ind] >= 2) {
        sl += a[ind];
        ind--;
    }
    sl += a[ind];

    if(a[k] == 1) {
        cout << max(sr, sl) + 1 << endl;
    } else {
        cout << sr + sl + a[k] << endl;
    }


}