Irreversible operation

AtCoder

IDagc029_a

Time2000ms

Memory256MB

Difficulty—

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. Consider performing the following operation: * 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. Find the maximum possible number of times this operation can be performed. ## Constraints * $1 \leq |S| \leq 2\times 10^5$ * $S_i=$`B` or `W` ## Input Input is given from Standard Input in the following format: $S$ [samples]

Samples

Input #1

BBW

Output #1

2

The operation can be performed twice, as follows:

*   Flip the second and third pieces from the left.
*   Flip the first and second pieces from the left.

Input #2

BWBWBW

Output #2

API Response (JSON)

{
  "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`, th...",
      "is_translate": false,
      "language": "English"
    }
  ]
}

Full JSON Raw Segments