#include <iostream>
#include <string>
using namespace std;

int main() {
  int N, K;
  string S;
  cin >> N >> K >> S;
  int l = 0, r = 0;
  if (S[K - 1] == '(')
    ++l;
  else
    ++r;
  int ans = -1;
  if (l) {
    for (int i = K; i <= N; ++i) {
      if (S[i] == '(')
        ++l;
      else
        ++r;

      if (l == r)
        ans = i + 1;
    }
  } else {
    for (int i = K - 2; i >= 0; --i) {
      if (S[i] == ')')
        ++r;
      else
        ++l;

      if (l == r)
        ans = i + 1;
    }
  }

  cout << ans << endl;
}