// #define _GLIBCXX_DEBUG // for STL debug (optional)
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <string>
#include <cstring>
#include <deque>
#include <list>
#include <queue>
#include <stack>
#include <vector>
#include <utility>
#include <algorithm>
#include <map>
#include <set>
#include <complex>
#include <cmath>
#include <limits>
#include <cfloat>
#include <climits>
#include <ctime>
#include <cassert>
#include <numeric>
#include <fstream>
#include <functional>
#include <bitset>
using namespace std;
using ll = long long int;
using int64 = long long int;
 
template<typename T> void chmax(T &a, T b) {a = max(a, b);}
template<typename T> void chmin(T &a, T b) {a = min(a, b);}
template<typename T> void chadd(T &a, T b) {a = a + b;}
 
int dx[] = {0, 0, 1, -1};
int dy[] = {1, -1, 0, 0};
const int INF = 1LL << 29;
const ll LONGINF = 1LL << 60;
const ll MOD = 1000000007LL;
 
int main() {
    int N, K; cin >> N >> K; K--;
    vector<ll> A(N);
    for(int i=0; i<N; i++) cin >> A[i];

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

        if(A[K] == 1) {
            cout << max(sumL, sumR) + A[K] << endl;
        }
        else {
            cout << sumL + sumR + A[K] << endl;
        }
    }
    return 0;
}