#ifndef TEMPLATE_HPP
#define TEMPLATE_HPP

#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")

#include <bits/stdc++.h>

#define impl_overload4(a, b, c, d, e, ...) e
#define impl_overload5(a, b, c, d, e, f, ...) f
#define impl_overload6(a, b, c, d, e, f, g, ...) g
#define impl_overload7(a, b, c, d, e, f, g, h, ...) h

// clang-format off
#define impl_rep4(i, a, b, c) for (int i = (a); i < (b); i += (c))
#define impl_rep3(i, a, b) impl_rep4(i, a, b, 1)
#define impl_rep2(i, n) impl_rep3(i, 0, n)
#define impl_rep1(n) for (int _ = 0; _ < (n); ++_)
#define rep(...) impl_overload4(__VA_ARGS__, impl_rep4, impl_rep3, impl_rep2, impl_rep1)(__VA_ARGS__)

#define impl_rrep4(i, a, b, c) for (int i = (b) - 1; i >= (a); i -= (c))
#define impl_rrep3(i, a, b) impl_rrep4(i, a, b, 1)
#define impl_rrep2(i, n) impl_rrep3(i, 0, n)
#define rrep(...) impl_overload4(__VA_ARGS__, impl_rrep4, impl_rrep3, impl_rrep2)(__VA_ARGS__)
// clang-format on

#define all(v) std::begin(v), std::end(v)
#define rall(v) std::rbegin(v), std::rend(v)

template<typename T>
constexpr int bit(T x, unsigned int k) {
  return (x >> k) & 1;
}

template<typename T>
constexpr bool chmax(T& a, const T& b) {
  return a < b ? a = b, true : false;
}
template<typename T>
constexpr bool chmin(T& a, const T& b) {
  return a > b ? a = b, true : false;
}

void yesno(bool b) {
  std::cout << (b ? "Yes" : "No") << "\n";
}
void yes() {
  yesno(true);
}
void no() {
  yesno(false);
}

struct Setup {
  Setup() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    std::cout << std::fixed << std::setprecision(11);
  }
} setup;

#ifdef LOCAL
#include "template_local.hpp"
#else
#define show(...) ((void)0)
#endif

using uint = unsigned int;
using lint = long long;
using ulint = unsigned long long;

using namespace std;

#endif // TEMPLATE_HPP

int main() {
  int n;
  string s, t;
  cin >> n >> s >> t;

  vector<int> a(n), b(n);
  rep (i, n) {
    a[i] = (s[i] == 'A' ? 0 : 1);
    b[i] = (t[i] == 'A' ? 0 : 1);
  }

  if (a[0] != b[0] || a[n - 1] != b[n - 1]) {
    cout << -1 << endl;
    return 0;
  }

  rep (i, n) {
    if (i % 4 == 2 || i % 4 == 3) {
      a[i] ^= 1;
      b[i] ^= 1;
    }
  }

  auto f = [](const vector<int>& s) -> vector<pair<int, int>> {
    int n = s.size();
    vector<pair<int, int>> res;
    rep (i, n) {
      if (s[i] == 0) continue;
      if (i == 0 || s[i - 1] == 0) {
        res.emplace_back(i, i + 1);
      }
      else {
        res.back().second++;
      }
    }
    return res;
  };

  auto la = f(a), lb = f(b);
  if (la.size() != lb.size()) {
    cout << -1 << endl;
    return 0;
  }

  lint ans = 0;
  rep (i, la.size()) {
    ans += abs(la[i].first - lb[i].first) + abs(la[i].second - lb[i].second);
  }
  cout << ans << endl;

  return 0;
}