#include #include using namespace std; using ll = long long; #define rep(i, s, t) for (ll i = s; i < (ll)(t); i++) #define all(x) begin(x), end(x) template bool chmin(T& x, T y) { return x > y ? (x = y, true) : false; } template bool chmax(T& x, T y) { return x < y ? (x = y, true) : false; } struct io_setup { io_setup() { ios::sync_with_stdio(false); cin.tie(nullptr); cout << fixed << setprecision(15); } } io_setup; template T sum_of_floor(T n, T m, T a, T b) { T ret = 0; if (a >= m) ret += (n - 1) * n * (a / m) / 2, a %= m; if (b >= m) ret += n * (b / m), b %= m; T y = (a * n + b) / m; if (y == 0) return ret; T x = y * m - b; ret += (n - (x + a - 1) / a) * y; ret += sum_of_floor(y, a, m, (a - x % a) % a); return ret; } using i128 = __int128_t; ll greedy(ll a, ll b, ll c, ll d) { ll res = 0; rep(i, 1, 10000) { ll x = (2 * a * i + b) / (2 * b); ll y = (2 * c * i + d) / (2 * d); if (x == y) { res++; cout << i << ' ' << x << endl; } } return res; } void solve() { ll a, b, c, d; cin >> a >> b >> c >> d; if (a * d == b * c) { cout << "-1\n"; return; } if (a * d > b * c) { swap(a, c); swap(b, d); } ll dv = 2 * (b * c - a * d); ll mx = (b * c + a * d + dv - 1) / dv; auto t1 = sum_of_floor(mx, 2 * c, 2 * d, d + 2 * c - 1); auto t2 = sum_of_floor(mx, 2 * a, 2 * b, -b + 2 * a - 1) + b / (2 * a) + 1; cout << (ll)(t1 - t2) << '\n'; } int main() { int t = 1; // cin >> t; while (t--) solve(); }