#include <iostream>
#include <vector>
#include <set>
#include <map>
#include <algorithm>
#include <iomanip>
#include <numeric>
#include <queue>
#include <stack>
using namespace std;
using ll = long long;
int main() {
	int N;cin >> N;
	string S, T;cin >> S >> T;
	vector<int> A(N), B(N);
	for (int i = 0;i < N;i++) {
		A[i] = S[i] - 'A';
		B[i] = T[i] - 'A';
	}
	if (A[0] != B[0] or A.back() != B.back()) {
		cout << -1 << endl;
		return 0;
	}
	vector<int> X1(N), Y1(N);
	for (int i = 0;i < N-1;i++) {
		X1[i] = A[i] ^ A[i + 1];
		Y1[i] = B[i] ^ B[i + 1];
	}
	vector<int> X(N), Y(N);
	for (int i = 0;i < N-1;i++) {
		if (i%2 == 0) {
			X[i] = 1 - X1[i];
			Y[i] = 1 - Y1[i];
		} else {
			X[i] = X1[i];
			Y[i] = Y1[i];
		}
	}
	queue<int> que;
	for (int i = 0;i < N-1;i++) if (Y[i] == 1) que.push(i);
	ll res = 0;
	for (int i = 0;i < N;i++) {
		if (X[i] == 1) {
			if (que.empty()) {
				cout << -1 << endl;
				return 0;
			}
			int id = que.front();
			que.pop();
			res += abs(id - i);
		}
	}
	if (!que.empty()) {
		cout << -1 << endl;
		return 0;
	}
	cout << res << endl;
}