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

int main()
{
	int N, K;
	string S;
	cin >> N >> K >> S;

	int ans = -1;
	K--;
	int s = 0;
	if( S[K] == '(' ) {
		for( int i = K; i < N; i++ ) {
			if( S[i] == '(' ) s++;
			else s--;
			if( s == 0 ) {
				ans = i + 1;
				break;
			}
		}
	}
	else {
		for( int i = K; i >= 0; i-- ) {
			if( S[i] == ')' ) s++;
			else s--;
			if( s == 0 ) {
				ans = i + 1;
				break;
			}
		}
	}

	cout << ans << endl;
}