#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
int main() {
	int n; cin >> n;
	string s, t; cin >> s >> t;
	rep(i, n) {
		if (i % 4 < 2) {
			if ('A' == s[i])s[i] = 'B';
			else s[i] = 'A';
			if ('A' == t[i])t[i] = 'B';
			else t[i] = 'A';
		}
	}
	if (s[0] != t[0]) {
		cout << -1 << endl;
		return 0;
	}
	vector<int>vs, vt;
	rep(i, n - 1) {
		if (s[i] != s[i + 1])vs.push_back(i);
		if (t[i] != t[i + 1])vt.push_back(i);
	}
	if (vs.size() != vt.size()) {
		cout << -1 << endl;
		return 0;
	}
	long long ans = 0;
	rep(i, vs.size()) {
		ans += abs(vs[i] - vt[i]);
	}
	cout << ans << endl;
	return 0;
}