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

const int INF = 1e9;
const int MOD = 1e9+7;
const ll LINF = 1e18;

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) for(int i=0;i<(n);++i)
#define REPR(i,n) for(int i=n;i>=0;i--)
#define ALL(v) (v.begin(),v.end())
#define COUT(x) cout<<(x)<<'\n'

template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
int main(){
    int n,k;
    cin >> n >> k;
    int a[n+1];
    ll ans = 0;
    REP(i,n){
        cin >>a[i+1];
    }
    ll num_r = 0,num_l=0;
    for(int i=k+1;i<n+1;i++){
        num_r += a[i];
        if(a[i]<2) break;
    }
    for(int i=k-1;i>0;i--){
        num_r += a[i];
        if(a[i]<2) break;
    }
    if(a[k]==0)ans = 0;
    else if(a[k]==1)ans = max(num_r,num_l)+1;
    else ans = num_r+num_l+a[k];
    
    COUT(ans);
    return 0;
}