#include <bits/stdc++.h>
using namespace std;

int main(){
    int N, K;
    cin >> N >> K;
    K--;
    vector<int> A(N);
    for(int i=0; i<N; i++) cin >> A[i];
    int64_t L = 0, R = 0;
    int pt = K-1;
    while(pt > 0 && A[pt] >= 2) pt--;
    if(pt >= 0) for(int i=pt; i<K; i++) L += A[i];
    pt = K+1;
    while(pt < N-1 && A[pt] >= 2) pt++;
    if(pt < N) for(int i=K+1; i<=pt; i++) R += A[i];
    int64_t ans = 0;
    if(A[K] == 0){
        ans = 0;
    }else if(A[K] == 1){
        ans = max(L, R) + 1;
    }else{
        ans = L+R+A[K];
    }
    cout << ans << endl;
    return 0;
}