#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll=long long;
using ld=long double;
ld pie=3.141592653589793;
ll inf=14449999999994;
ll mod=1000000007;
int main(){
    ll n,k;
    cin >> n >> k;
    vector<ll>a(n);
    for (ll i = 0; i < n; i++)
    {
        cin >> a[i];
    }
    k--;
    ll x=0,y=0;
    for (ll i = k+1; i < n; i++)
    {
        if (a[i]<=1)
        {
            x+=a[i];
            break;
        }
        x+=a[i];
    }
    for (ll i = k-1; i >=0; i--)
    {
        if (a[i]<=1)
        {
            y+=a[i];
            break;
        }
        y+=a[i];
    }
    if (a[k]>=2)
    {
        cout << x+y+a[k] << endl;
    }else if (a[k]==1)
    {
        cout << max(x,y)+1 << endl;
    }else{
        cout << 0 << endl;
    }
}