#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<bitset>
#include<set>
#include<map>
#include<stack>
#include<queue>
#include<deque>
#include<list>
#include<iomanip>
#include<cmath>
#include<cstring>
#include<functional>
#include<cstdio>
#include<cstdlib>
using namespace std;

#define repr(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define rep(i, n) repr(i, 0, n)
#define INF 2e9
#define MOD 1000000007
//#define MOD 998244353
#define LINF (long long)4e18
#define jck 3.141592

const double EPS = 1e-10;

using ll = long long;
using Pi = pair<int,int>;
using Pl = pair<ll,ll>;



int main(){
    int N,K; cin >> N >> K;
    K--;
    vector<int> a(N);
    rep(i,N) cin >> a[i];
    if(a[K] == 0){
        cout << 0 << endl;
        return 0;
    }
    int idx1 = N-1,idx2 = 0;
    repr(i,K+1,N){
        if(a[i] == 1){
            idx1 = i;
            break;
        }
        if(a[i] == 0){
            idx1 = i-1;
            break;
        }
    }
    for(int i = K-1; i >= 0; i--){
        if(a[i] == 1){
            idx2 = i;
            break;
        }
        if(a[i] == 0){
            idx2 = i+1;
            break;
        }
    }
    ll sum1 = a[K],sum2 = a[K];
    repr(i,K+1,idx1+1){
        sum1 += a[i];
    }
    for(int i = K-1; i >= idx2; i--){
        sum2 += a[i];
    }
    if(a[K] == 1) cout << max(sum1,sum2) << endl;
    else cout << sum1+sum2-a[K] << endl;
    
}