#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    int N,K; cin >> N >> K; K--;
    vector<ll> A(N);
    rep(i,N) cin >> A[i];

    if(A[K] == 0) {
        cout << 0 << endl;
    } else {
        int L = K;
        while(0 <= L && A[L] >= 2) L--;
        int R = K;
        while(R <  N && A[R] >= 2) R++;

        ll sumL = 0, sumR = 0;
        for(int i = L; i <= K; i++) sumL += A[i];
        for(int i = K; i <= R; i++) sumR += A[i];
        if(A[K] == 1) {
            cout << max(sumL, sumR) << endl;
        } else {
            cout << sumL + sumR - A[K] << endl;
        }
    }
}