#include<bits/stdc++.h>

using namespace std;

#define INF 1000000007
#define LINF (1LL << 62)

typedef long long i64;
typedef pair<i64,i64> P;

inline i64 mod(i64 a, i64 m) { return (a % m + m) % m; }

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }



int n, k, a[202020];
void solve(){
	cin >> n >> k;
	for(int i = 0; i < n; i++){
		cin >> a[i];
	}
	k--;
	
	if(a[k] == 0){
		cout << 0 << endl;
		return;
	}
	i64 l = 0, r = 0;
	for(int i = k-1; i >= 0; i--){
		if(a[i] == 0) break;
		l += a[i];
		if(a[i] == 1) break;
	}
	for(int i = k+1; i < n; i++){
		if(a[i] == 0) break;
		r += a[i];
		if(a[i] == 1) break;
	}
	if(a[k] == 1){
		cout << 1+max(l,r) << endl;
	}
	else{
		cout << a[k]+l+r << endl;
	}
}

int main(){
	std::cin.tie(0);
	std::ios::sync_with_stdio(false);
	int t = 1;
	//cin >> t;
	while(t--){
		solve();
	}
	
	return 0;
}