#include <iostream>
using namespace std;

int n, k, i, j;

int main(){
    cin >> n >> k;
    int p, q;
    int a[n];
    int ans = 0;
    for(i = 0; i < n; i++){cin >> a[i];}
    
    if(a[k - 1] == 0){ 
        cout << 0 << endl;
        return 0;
    }
    
    p = 0, q = n - 1;
    for(i = 0; i < n; i++){
        if(i < k){
            if(a[i] == 0) p = i + 1;
            if(a[i] == 1) p = i;
        }else{
            if(a[i] == 0) q = i - 1;
            if(a[i] == 1) q = i;
        }
    }
    
    for(i = p; i < q + 1; i++){
        ans += a[i];
    }
    
    cout << ans << endl;
    
    return 0;
}