#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

#define INF (long long)(1e9+7)
#define REP(i,n) for(ll i = 0; i < n; i++)
#define REP1(i,n) for(ll i = 1; i <= n; i++)

ll a[200020];

int main(void)
{
    int n, k;
    ll cntd = 0, cntu = 0, cnt;
    cin >> n >> k;
    k--;
    REP(i, n) cin >> a[i];
    if(a[k] == 0) {
        cout << 0 << endl;
        return 0;
    }
    for(int i = k-1; i >= 0; i--) {
        if(a[i]==0) break;
        cntd += a[i];
        if(a[i] <= 1) break;
    }
    for(int i = k + 1; i < n; i++) {
        cntu += a[i];
        if(a[i] <= 1) break;
    }
    if(a[k] == 1) {
        cnt = max(cntu, cntd) +1;
    } else {
        cnt = cntu + cntd + a[k];
    }
    cout << cnt << endl;
}