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

int main() {
	int n, k; cin >> n >> k;
	--k;
	int a[n]; for(int i = 0; i < n; ++i) cin >> a[i];
	if(a[k] == 0) {
		cout << 0 << '\n';
		return 0;
	}
	int64_t l = 0;
	for(int i = k - 1; i >= 0; --i) {
		l += a[i];
		if(a[i] <= 1) break;
	}
	int64_t r = 0;
	for(int i = k + 1; i < n; ++i) {
		r += a[i];
		if(a[i] <= 1) break;
	}
	if(a[k] == 1) {
		cout << max(l, r) + a[k] << '\n';
		return 0;
	}
	cout << l + r + a[k] << '\n';
	return 0;
}