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

using ll = long long;
using ull = unsigned long long;
using ld = long double;
template<class t> using vc = vector<t>;
template<class t> using vvc = vc<vc<t>>;
using pi = pair<int,int>;
using vi = vc<int>;
using vvi = vvc<int>;

#define rep(i,a,b) for (int i = a; i < b; i++)
#define irep(i,a,b) for (int i = a; i > b; i--)
#define print(n) cout << n << '\n'
#define pritn(n) print(n)
#define rup(a,b) (a+b-1)/b
#define input(A,N) rep(i,0,N) cin>>A[i]
#define chmax(a,b) a = max(a,b)
#define chmin(a,b) a = min(a,b)


int main(){
    cout << fixed << setprecision(15);
    
    ll N,K;
    cin>>N>>K;
    K--;
    vc<ll> A(N);
    input(A,N);
    ll left = 0;
    ll right = 0;
    for(int i = K-1;i>=0;i--){
        left += A[i];
        if (A[i]<=1) break;
    }
    for(int i = K+1;i<N;i++){
        right += A[i];
        if(A[i]<=1) break;
    }
    if (A[K]>=2){
        print(right+left+A[K]);
    }else if(A[K]==1){
        print(max(right,left)+A[K]);
    }else{
        print(0);
    }
    
    //system("pause");
    return 0;
}