#include <bits/stdc++.h>
using namespace std::literals::string_literals;
using i64 = std::int_fast64_t;
using std::cout;
using std::endl;
using std::cin;

template<typename T>
std::vector<T> make_v(size_t a){return std::vector<T>(a);}

template<typename T,typename... Ts>
auto make_v(size_t a,Ts... ts){
  return std::vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));
}

int main() {
	int n, k; scanf("%d%d", &n, &k); k--; std::vector<i64> a(n);
	for(int i = 0; i < n; i++) scanf("%lld", &a[i]);
	
	if(!a[k]) {
		printf("0\n");
		return 0;
	}
	
	i64 ans = a[k];
	for(int i = k; i + 1 < n; i++) {
		if(a[i + 1] == 1) {
			ans += 1;
			break;
		}
		ans += a[i + 1];
	}
	for(int i = k; i - 1 >= 0; i--) {
		if(a[i - 1] == 1) {
			ans += 1;
			break;
		}
		ans += a[i - 1];
	}
	
	printf("%lld\n", ans);
	return 0;
}