#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)

using std::cin;
using std::cout;
using std::vector;
using std::string;
using ll = long long;

int main() {
    int n;
    cin >> n;
    string s, t;
    cin >> s >> t;
    
    rep(i, n) if (i%4 >= 2){
        s[i] ^= 'A'^'B';
        t[i] ^= 'A'^'B';
    }
    
    if (s[0] != t[0] or s.back() != t.back()) {
        puts("-1");
        return 0;
    }
    
    vector<int> pos_s, pos_t;
    rep(i, n) {
        if (s[i] != s[i+1]) pos_s.push_back(i);
        if (t[i] != t[i+1]) pos_t.push_back(i);
    }
    
    if (pos_s.size() != pos_t.size()) {
        puts("-1");
        return 0;
    }
    
    ll ans = 0;
    rep(i, pos_s.size()) {
        ans += abs(pos_s[i]-pos_t[i]);
    }
    
    cout << ans << '\n';
    
    return 0;
}