#include <bits/stdc++.h>
using namespace std;
int main(){
  int N;
  cin >> N;
  string S;
  cin >> S;
  string T;
  cin >> T;
  for (int i = 0; i < N; i++){
    if (i % 4 >= 2){
      S[i] ^= 'A' ^ 'B';
      T[i] ^= 'A' ^ 'B';
    }
  }
  if (S[0] != T[0] || S[N - 1] != T[N - 1]){
    cout << -1 << endl;
  } else {
    vector<int> p, q;
    for (int i = 0; i < N - 1; i++){
      if (S[i] != S[i + 1]){
        p.push_back(i);
      }
      if (T[i] != T[i + 1]){
        q.push_back(i);
      }
    }
    if (p.size() != q.size()){
      cout << -1 << endl;
    } else {
      int cnt = p.size();
      long long ans = 0;
      for (int i = 0; i < cnt; i++){
        ans += abs(p[i] - q[i]);
      }
      cout << ans << endl;
    }
  }  
}