#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
template<class T> using V = vector<T>;
using VI = V<int>;
using VL = V<ll>;
using VS = V<string>;
template<class T> using PQ = priority_queue<T, V<T>, greater<T>>;
using graph = V<VI>;
template<class T> using w_graph = V<V<pair<int, T>>>;
#define FOR(i,a,n) for(int i=(a);i<(n);++i)
#define eFOR(i,a,n) for(int i=(a);i<=(n);++i)
#define rFOR(i,a,n) for(int i=(n)-1;i>=(a);--i)
#define erFOR(i,a,n) for(int i=(n);i>=(a);--i)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define inside(h,w,y,x) (unsigned(y)<h&&unsigned(x)<w)
#ifdef _DEBUG
#define line cout << "-----------------------------\n"
#define stop system("pause")
#endif
constexpr ll INF = 1000000000;
constexpr ll LLINF = 1LL << 61;
constexpr ll mod = 1000000007;
constexpr ll MOD = 998244353;
constexpr ld eps = 1e-10;
constexpr int dy[]{ -1,0,1,0 }, dx[]{ 0,1,0,-1 };
template<class T> inline bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; }return false; }
template<class T> inline bool chmin(T& a, const T& b) { if (a > b) { a = b; return true; }return false; }
inline void init() { cin.tie(nullptr); cout.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); }
template<class T> inline istream& operator>>(istream& is, V<T>& v) { for (auto& a : v)is >> a; return is; }
template<class T, class U> inline istream& operator>>(istream& is, pair<T, U>& p) { is >> p.first >> p.second; return is; }
template<class T> inline V<T> vec(size_t a) { return V<T>(a); }
template<class T> inline V<T> defvec(T def, size_t a) { return V<T>(a, def); }
template<class T, class... Ts> inline auto vec(size_t a, Ts... ts) { return V<decltype(vec<T>(ts...))>(a, vec<T>(ts...)); }
template<class T, class... Ts> inline auto defvec(T def, size_t a, Ts... ts) { return V<decltype(defvec<T>(def, ts...))>(a, defvec<T>(def, ts...)); }
template<class T> inline void print(const T& a) { cout << a << "\n"; }
template<class T, class... Ts> inline void print(const T& a, const Ts&... ts) { cout << a << " "; print(ts...); }
template<class T> inline void print(const V<T>& v) { for (int i = 0; i < v.size(); ++i)cout << v[i] << (i == v.size() - 1 ? "\n" : " "); }
template<class T> inline void print(const V<V<T>>& v) { for (auto& a : v)print(a); }
template<class T> inline constexpr const T cumsum(const V<T>& a, int l, int r) { return 0 <= l && l <= r && r < a.size() ? a[r] - (l == 0 ? 0 : a[l - 1]) : 0; }//[l,r]
template<class T> inline constexpr const T min(const V<T>& v) { return *min_element(all(v)); }
template<class T> inline constexpr const T max(const V<T>& v) { return *max_element(all(v)); }
template<class T> inline V<T>& operator++(V<T>& v) { for (T& a : v)++a; return v; }
template<class T> inline V<T>& operator--(V<T>& v) { for (T& a : v)--a; return v; }

VI dif(VI a, bool fix) {
    if (fix) {
        a.insert(a.begin(), 0);
        a.push_back(0);
    }
    int n = a.size();
    VI b(n - 1);
    FOR(i, 0, n - 1)b[i] = a[i] != a[i + 1];
    return b;
}

template<class C> class TestcaseGenerator {
    random_device rnd;
    mt19937 mt;
public:
    TestcaseGenerator() : mt(mt19937(rnd())) {}
    C integer(C value_min, C value_max) {
        return uniform_int_distribution<C>(value_min, value_max)(mt);
    }
    V<C> sequence(int n, C value_min, C value_max) {
        V<C> a(n);
        uniform_int_distribution<C> value(value_min, value_max);
        FOR(i, 0, n)a[i] = value(mt);
        return a;
    }
    VI permutation(int n) {
        VI p(n);
        FOR(i, 0, n)p[i] = i;
        FOR(j, 0, 3)FOR(i, 1, n)swap(p[i], p[uniform_int_distribution<int>(0, i - 1)(mt)]);
        return p;
    }
    graph tree(int n) {
        graph g(n);
        FOR(i, 1, n) {
            int j = uniform_int_distribution<int>(0, i - 1)(mt);
            g[i].push_back(j);
            g[j].push_back(i);
        }
        return g;
    }
    w_graph<C> weighted_tree(int n, C weight_max) {
        w_graph<C> g(n);
        uniform_int_distribution<C> weight(1, weight_max);
        FOR(i, 1, n) {
            int j = uniform_int_distribution<int>(0, i - 1)(mt);
            C w = weight(mt);
            g[i].emplace_back(j, w);
            g[j].emplace_back(i, w);
        }
        return g;
    }
};
pair<string, string> g() {
    TestcaseGenerator<int> gen;
    int n = 100000;
    VI p = gen.sequence(n, 0, 1);
    string s(n, '?');
    FOR(i, 0, n)s[i] = "AB"[p[i]];
    string t = s;
    FOR(_, 0, n) {
        int i = gen.integer(0, n - 3);
        if (p[i] == p[i + 2]) {
            p[i] ^= 1;
            p[i + 2] ^= 1;
        }
    }
    FOR(i, 0, n)s[i] = "AB"[p[i]];
    return {s,t};
}

int main() {
    init();

    int n; cin >> n;
    string s, t; cin >> s >> t;
    if (s[0] != t[0] || s.back() != t.back()) {
        print(-1);
        return 0;
    }
    VI a(n), b(n);
    FOR(i, 0, n) {
        a[i] = s[i] == 'B';
        b[i] = t[i] == 'B';
    }
    a = dif(a, 0);
    b = dif(b, 0);
    int ans = 0;
    stack<int> st;
    FOR(i, 0, a.size()) {
        if (i + 1 < a.size() && a[i] != b[i]) {
            ++ans;
            if (a[i] == a[i + 1]) {
                a[i] ^= 1;
                a[i + 1] ^= 1;
            }
            else st.push(i);
        }
        else {
            while (!st.empty()) {
                int j = st.top();
                st.pop();
                if (a[j] != a[j + 1]) {
                    break;
                }
                a[j] ^= 1;
                a[j + 1] ^= 1;
            }
        }
    }
    while (!st.empty()) {
        int j = st.top();
        st.pop();
        if (a[j] != a[j + 1]) {
            break;
        }
        a[j] ^= 1;
        a[j + 1] ^= 1;
    }
    print(a == b ? ans : -1);

    return 0;
}