#include <algorithm>
#include <iostream>
#include <numeric>
#include <string>
#include <tuple>
#include <utility>
#include <vector>
#include <limits>

#define rep(i, a, b) for (int i = int(a); i < int(b); i++)
using namespace std;
using ll = long long int;  // NOLINT
using P = pair<ll, ll>;

// clang-format off
#ifdef _DEBUG_
#define dump(...) do{ cerr << __LINE__ << ":\t" << #__VA_ARGS__ << " = "; debug_print(__VA_ARGS__); } while(false)
template<typename T, typename... Ts> void debug_print(const T &t, const Ts &...ts) { cerr << t; ((cerr << ", " << ts), ...); cerr << endl; }
#else
#define dump(...) do{ } while(false)
#endif
template<typename T> vector<T> make_v(size_t a, T b) { return vector<T>(a, b); }
template<typename... Ts> auto make_v(size_t a, Ts... ts) { return vector<decltype(make_v(ts...))>(a, make_v(ts...)); }
template<typename T> bool chmin(T &a, const T& b) { if (a > b) {a = b; return true; } return false; }
template<typename T> bool chmax(T &a, const T& b) { if (a < b) {a = b; return true; } return false; }
template<typename T, typename... Ts> void print(const T& t, const Ts&... ts) { cout << t; ((cout << ' ' << ts), ...); cout << '\n'; }
template<typename... Ts> void input(Ts&... ts) { (cin >> ... >> ts); }
template<typename T> istream &operator,(istream &in, T &t) { return in >> t; }
struct Inf { template<typename T> constexpr operator T() { return numeric_limits<T>::max() / 2; } };
// clang-format on

char rev(char c) {
    return c == 'A' ? 'B' : 'A';
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n;
    string s, t;
    cin, n, s, t;
    int ans = 0;

    rep(i, 0, n - 1) {
        if (s[i] == t[i]) continue;
        if (i && s[i - 1] == s[i + 1]) {
            s[i] = rev(s[i]);
            ans++;
        } else if (i + 2 < n && s[i] == s[i + 2]) {
            s[i + 1] = rev(s[i + 1]);
            s[i] = rev(s[i]);
            ans += 2;
        }
    }

    if (s != t) ans = -1;
    print(ans);
    return 0;
}