#include <iostream>
#include <vector>
#include <string>
#include <cmath>

using namespace std;
int main(){
    int i,n; cin >> n;
    string s,t; cin >> s >> t;
    if(s[0]!=t[0] || s.back()!=t.back()){
        cout << "-1\n";
        return 0;
    }
    vector<int> s1,t1;
    for(i=1;i<n;i++){
        if((i&1)^(s[i - 1]==s[i])) s1.push_back(i);
        if((i&1)^(t[i - 1]==t[i])) t1.push_back(i);
    }
    if(s1.size()!=t1.size()){
        cout << "-1\n";
        return 0;
    }
    int ans = 0;
    for(i=0;i<s1.size();i++){
        ans += abs(s1[i] - t1[i]);
    }
    cout << ans << "\n";
}