#define _USE_MATH_DEFINES #include using namespace std; #define FOR(i,m,n) for(int i=(m);i<(n);++i) #define REP(i,n) FOR(i,0,n) #define ALL(v) (v).begin(),(v).end() using ll = long long; constexpr int INF = 0x3f3f3f3f; constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL; constexpr double EPS = 1e-8; constexpr int MOD = 1000000007; // constexpr int MOD = 998244353; constexpr int DY4[]{1, 0, -1, 0}, DX4[]{0, -1, 0, 1}; constexpr int DY8[]{1, 1, 0, -1, -1, -1, 0, 1}; constexpr int DX8[]{0, -1, -1, -1, 0, 1, 1, 1}; template inline bool chmax(T& a, U b) { return a < b ? (a = b, true) : false; } template inline bool chmin(T& a, U b) { return a > b ? (a = b, true) : false; } struct IOSetup { IOSetup() { std::cin.tie(nullptr); std::ios_base::sync_with_stdio(false); std::cout << fixed << setprecision(20); } } iosetup; template struct FenwickTree { explicit FenwickTree(const int n, const Abelian ID = 0) : n(n), ID(ID), data(n, ID) {} void add(int idx, const Abelian val) { for (; idx < n; idx |= idx + 1) { data[idx] += val; } } Abelian sum(int idx) const { Abelian res = ID; for (--idx; idx >= 0; idx = (idx & (idx + 1)) - 1) { res += data[idx]; } return res; } Abelian sum(const int left, const int right) const { return left < right ? sum(right) - sum(left) : ID; } Abelian operator[](const int idx) const { return sum(idx, idx + 1); } int lower_bound(Abelian val) const { if (val <= ID) return 0; int res = 0, exponent = 1; while (exponent <= n) exponent <<= 1; for (int mask = exponent >> 1; mask > 0; mask >>= 1) { const int idx = res + mask - 1; if (idx < n && data[idx] < val) { val -= data[idx]; res += mask; } } return res; } private: const int n; const Abelian ID; std::vector data; }; template long long inversion_number(const std::vector& a) { const int n = a.size(); std::vector b = a; std::sort(b.begin(), b.end()); b.erase(std::unique(b.begin(), b.end()), b.end()); FenwickTree bit(b.size()); long long res = 0; for (int i = 0; i < n; ++i) { const int idx = std::distance( b.begin(), std::lower_bound(b.begin(), b.end(), a[i])); res += i - bit.sum(idx + 1); bit.add(idx, 1); } return res; } // https://codeforces.com/contest/1705/problem/D int main() { int n; string s, t; cin >> n >> s >> t; if (s.front() != t.front() || s.back() != t.back()) { cout << "-1\n"; return 0; } vector sd, td; sd.reserve(n - 1); td.reserve(n - 1); FOR(i, 1, n) sd.emplace_back((s[i - 1] - 'A') ^ (s[i] - 'A')); FOR(i, 1, n) td.emplace_back((t[i - 1] - 'A') ^ (t[i] - 'A')); if (count(ALL(sd), 0) != count(ALL(td), 0)) { cout << "-1\n"; return 0; } vector p[2]{}; for (int i = n - 2; i >= 0; --i) p[sd[i]].emplace_back(i); vector a(n - 1); REP(i, n - 1) { a[p[td[i]].back()] = i; p[td[i]].pop_back(); } cout << inversion_number(a) << '\n'; return 0; }