#include <bits/stdc++.h>
using namespace std;
#include <atcoder/modint>
using mint = atcoder::modint998244353;

#define rep(i, l, r) for (int i = (int)(l); i < (int)(r); i++)
#define ll long long
#define all(x) (x).begin(), (x).end()
#define siz(x) (int)x.size()

const int inf = 1e9;
const ll INF = 4e18;

template<class T> bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }

int main() {
    int N; string S, T; 
    cin >> N >> S >> T;
    
    if (S[0] != T[0] or S[N-1] != T[N-1]) {
        //NG
        cout << -1 << endl;
        return 0;
    }
    
    vector<int> A(N - 1), B(N - 1);
    rep(i, 0, N-1) {
        A[i] = (S[i] != S[i+1]);
        B[i] = (T[i] != T[i+1]);
    }
    
    //偶数bitを反転（天才？）
    
    rep(i, 0, N-1) {
        if (i % 2 == 0) {
            A[i] = 1 - A[i];
            B[i] = 1 - B[i];
        }
    }
    
    vector<int> P, Q;
    rep(i, 0, N - 1) {
        if (A[i] == 1) P.push_back(i);
        if (B[i] == 1) Q.push_back(i);
    }
    
    if (siz(P) != siz(Q)) {
        cout << -1 << endl;
        return 0;
    }
    
    ll answer = 0;
    rep(i, 0, siz(P)) answer += abs(P[i] - Q[i]);
    
    cout << answer << endl;
}