{"problem":{"name":"Irreversible operation","description":{"content":"There are $N$ Reversi pieces arranged in a row. (A _Reversi piece_ is a disc with a black side and a white side.) The state of each piece is represented by a string $S$ of length $N$. If $S_i=$`B`, th","description_type":"Markdown"},"platform":"AtCoder","limit":{"time_limit":2000,"memory_limit":262144},"difficulty":"None","is_remote":true,"is_sync":true,"sync_url":null,"sign":"agc029_a"},"statements":[{"statement_type":"Markdown","content":"There are $N$ Reversi pieces arranged in a row. (A _Reversi piece_ is a disc with a black side and a white side.) The state of each piece is represented by a string $S$ of length $N$. If $S_i=$`B`, the $i$\\-th piece from the left is showing black; If $S_i=$`W`, the $i$\\-th piece from the left is showing white.\nConsider performing the following operation:\n\n*   Choose $i$ ($1 \\leq i < N$) such that the $i$\\-th piece from the left is showing black and the $(i+1)$\\-th piece from the left is showing white, then flip both of those pieces. That is, the $i$\\-th piece from the left is now showing white and the $(i+1)$\\-th piece from the left is now showing black.\n\nFind the maximum possible number of times this operation can be performed.\n\n## Constraints\n\n*   $1 \\leq |S| \\leq 2\\times 10^5$\n*   $S_i=$`B` or `W`\n\n## Input\n\nInput is given from Standard Input in the following format:\n\n$S$\n\n[samples]","is_translate":false,"language":"English"}],"meta":{"iden":"agc029_a","tags":[],"sample_group":[["BBW","2\n\nThe operation can be performed twice, as follows:\n\n*   Flip the second and third pieces from the left.\n*   Flip the first and second pieces from the left."],["BWBWBW","6"]],"created_at":"2026-03-03 11:01:13"}}