#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint1000000007;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000000



int main(){
	
	int n;
	cin>>n;
	
	string s,t;
	cin>>s>>t;
	
	rep(i,n){
		if(i%4<=1){
			if(s[i]=='A')s[i] = 'B';
			else s[i] = 'A';
			if(t[i]=='A')t[i]=  'B';
			else t[i] = 'A';
		}
	}
	
	if(s[0]!=t[0]||s[n-1]!=t[n-1]){
		cout<<-1<<endl;
		return 0;
	}
	vector<int> a,b;
	rep(i,n-1){
		if(s[i]!=s[i+1])a.push_back(i);
		if(t[i]!=t[i+1])b.push_back(i);
	}
	if(a.size()!=b.size()){
		cout<<-1<<endl;
		return 0;
	}
	long long ans = 0;
	rep(i,a.size())ans += abs(a[i]-b[i]);
	
	cout<<ans<<endl;
			
    return 0;
}