{"problem":{"name":"Billiards","description":{"content":"Takahashi is playing billiards on a two-dimensional plane. The $x$\\-axis functions as a wall; when a ball hits the axis, it will bounce off the axis so that the angle of incidence equals the angle of ","description_type":"Markdown"},"platform":"AtCoder","limit":{"time_limit":2000,"memory_limit":262144},"difficulty":"None","is_remote":true,"is_sync":true,"sync_url":null,"sign":"abc183_b"},"statements":[{"statement_type":"Markdown","content":"Takahashi is playing billiards on a two-dimensional plane. The $x$\\-axis functions as a wall; when a ball hits the axis, it will bounce off the axis so that the angle of incidence equals the angle of reflection.\nTakahashi's ball is now at $(S_x,S_y)$. When he strikes the ball aiming for some point, it will roll in a straight line towards that point.\nTo make the ball hit the $x$\\-axis exactly once and then pass $(G_x, G_y)$, where along the $x$\\-axis should he aim for?\n\n## Constraints\n\n*   $-10^6 \\leq S_x, G_x \\leq 10^6$\n*   $0 < S_y, G_y \\leq 10^6$\n*   $S_x \\neq G_x$\n*   All values in input are integers.\n\n## Input\n\nInput is given from Standard Input in the following format:\n\n$S_x$ $S_y$ $G_x$ $G_y$\n\n[samples]","is_translate":false,"language":"English"}],"meta":{"iden":"abc183_b","tags":[],"sample_group":[["1 1 7 2","3.0000000000\n\nAs shown below, we can make the ball pass $(7, 2)$ by striking it aiming for $(3, 0)$.\n![image](https://img.atcoder.jp/ghi/c9595d59f1139b808d4cf3d31d6b48ee.png)"],["1 1 3 2","1.6666666667\n\n![image](https://img.atcoder.jp/ghi/4f2c808fddc9bb349999f8969996ebb9.png)"],["\\-9 99 -999 9999","\\-18.7058823529\n\nThe output will be considered correct when its absolute or relative error from our answer is at most $10^{-6}$."]],"created_at":"2026-03-03 11:01:14"}}