#include using namespace std; using ll = long long; using ld = long double; template using V = vector; using VI = V; using VL = V; using VS = V; template using PQ = priority_queue, greater>; using graph = V; template using w_graph = V>>; #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) inline bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; }return false; } template 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 inline istream& operator>>(istream& is, V& v) { for (auto& a : v)is >> a; return is; } template inline istream& operator>>(istream& is, pair& p) { is >> p.first >> p.second; return is; } template inline V vec(size_t a) { return V(a); } template inline V defvec(T def, size_t a) { return V(a, def); } template inline auto vec(size_t a, Ts... ts) { return V(ts...))>(a, vec(ts...)); } template inline auto defvec(T def, size_t a, Ts... ts) { return V(def, ts...))>(a, defvec(def, ts...)); } template inline void print(const T& a) { cout << a << "\n"; } template inline void print(const T& a, const Ts&... ts) { cout << a << " "; print(ts...); } template inline void print(const V& v) { for (int i = 0; i < v.size(); ++i)cout << v[i] << (i == v.size() - 1 ? "\n" : " "); } template inline void print(const V>& v) { for (auto& a : v)print(a); } template inline constexpr const T cumsum(const V& 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 inline constexpr const T min(const V& v) { return *min_element(all(v)); } template inline constexpr const T max(const V& v) { return *max_element(all(v)); } template inline V& operator++(V& v) { for (T& a : v)++a; return v; } template inline V& operator--(V& 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 TestcaseGenerator { random_device rnd; mt19937 mt; public: TestcaseGenerator() : mt(mt19937(rnd())) {} C integer(C value_min, C value_max) { return uniform_int_distribution(value_min, value_max)(mt); } V sequence(int n, C value_min, C value_max) { V a(n); uniform_int_distribution 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(0, i - 1)(mt)]); return p; } graph tree(int n) { graph g(n); FOR(i, 1, n) { int j = uniform_int_distribution(0, i - 1)(mt); g[i].push_back(j); g[j].push_back(i); } return g; } w_graph weighted_tree(int n, C weight_max) { w_graph g(n); uniform_int_distribution weight(1, weight_max); FOR(i, 1, n) { int j = uniform_int_distribution(0, i - 1)(mt); C w = weight(mt); g[i].emplace_back(j, w); g[j].emplace_back(i, w); } return g; } }; pair g() { TestcaseGenerator 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); VI x, y; FOR(i, 0, a.size()) { if (a[i] ^ (i&1))x.push_back(i); if (b[i] ^ (i&1))y.push_back(i); } if (x.size() != y.size()) { print(-1); return 0; } ll ans = 0; FOR(i, 0, x.size())ans += abs(x[i] - y[i]); print(ans); return 0; }