#include <algorithm>
#include <cassert>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <iostream>
#include <tuple>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <ranges>
#include <iomanip>
#include <bitset>
using namespace std;
using ll = long long;

auto range(int n) { return views::iota(0, n); }
#define repeat(n) for ([[maybe_unused]] auto : range(n))

template<class T, class U> ostream& operator<<(ostream& os, const pair<T, U>& p){ return os << "{" << p.first << ", " << p.second << "}"; }
template<typename T> ostream& operator<<(ostream& os, const vector<T>& obj) { os << "{"; for (const auto& e : obj) os << e << ", "; return os << "}"; }
template<typename T> ostream& operator<<(ostream& os, const set<T>& obj) { os << "set{"; for (const auto& e : obj) os << e << ", "; return os << "}"; }
template<typename T, typename U> ostream& operator<<(ostream& os, const map<T, U>& obj) { os << "map{"; for (const auto& e : obj) os << e << ", "; return os << "}"; }

#ifdef ONLINE_JUDGE
#define dump(expr) ;
#else
#define dump(expr) { cerr << "\033[33m#L" << __LINE__ << ": " << expr << "\033[39m" << endl; }
#endif

struct UFWithWeights {
  vector<int> data, weights;
  UFWithWeights(int n) : data(n, -1), weights(n, 1) {}
  int root(int x) { return data[x] < 0 ? x : data[x] = root(data[x]); }
  bool find(int x, int y) { return root(x) == root(y); }
  void uni(int x, int y) {
    x = root(x);
    y = root(y);
    if (x != y) {
      int new_weight = weights[x] + weights[y];
      if (data[x] < data[y])
        swap(x, y);
      data[x] += data[y];
      data[y] = x;
      weights[x] = new_weight;
    }
  }
  int weight(int x) { return weights[root(x)]; }
};

int get(int x, int i) { return (x >> i) & 1; }

void experiment() {
  const int n = 11;
  UFWithWeights uf(1 << n);

  for (int x : range(1 << n)) {
    for (int i : range(n - 2)) {
      if (get(x, i) == get(x, i + 2)) {
        int y = x ^ (1 << (i + 1));
        uf.uni(x, y);
      }
    }
  }

  vector<vector<int>> vss(1 << n);
  for (int x : range(1 << n)) vss[uf.root(x)].push_back(x);
  for (auto vs : vss) if (vs.size() > 0) {
    int m = *min(vs.begin(), vs.end());
    // cout << vs << endl;
    auto s = bitset<n>(m).to_string();
    if (s[0] == '0' && s[n - 1] == '0') {
      for (auto x : vs) cout << bitset<n>(x).to_string() << " ";
      cout << endl;
    }
  }
}

ll solve() {
  int n; cin >> n;
  string s_in, t_in; cin >> s_in >> t_in;
  if (s_in[0] != t_in[0]) return -1;
  if (s_in[n - 1] != t_in[n - 1]) return -1;

  vector<int> s(n - 1), t(n - 1);
  for (int i : range(n - 1)) {
    s[i] = (s_in[i] - 'A') ^ (s_in[i + 1] - 'A');
    t[i] = (t_in[i] - 'A') ^ (t_in[i + 1] - 'A');
  }
  dump(s);
  dump(t);
  int i = 0, j = 0;
  ll res = 0;
  while (i < n - 1 && j < n - 1) {
    // In [0, i), s and t are the same
    // In [i, j), s and t are not the same and s is alternating
    if (s[i] == t[i]) {
      i++;
      continue;
    }
    assert(s[i] != t[i]);

    if (j < i) j = i;

    if (j - 1 >= i && s[j] == s[j - 1]) {
      // i      j
      // 01010100
      // ->
      // 11010101
      res += j - i;
      s[i] ^= 1;
      s[j] ^= 1;
    } else {
      j++;
    }
  }
  if (i == n - 1) return res;
  else return -1;
}

int main() {
  cout << fixed << setprecision(12);
  // experiment();
  cout << solve() << endl;
}