TopCoder SRM614: TorusSailingEasy

解法については、 TopCoder SRM614 解説 を参考にしてください。

public class TorusSailingEasy {
    int reach(int N, int M, int x, int y) {
        for (int p = x; p < N * M; p += N) if (p % M == y) return p;
        return -1;
    }

    public double expectedTime(int N, int M, int goalX, int goalY) {
        var r1 = reach(N, M, goalX, goalY);
        var r2 = reach(N, M, (N - goalX) % N, (M - goalY) % M);
        if (r1 == -1 || r2 == -1) return -1;
        return r1 * r2;
    }
}