Alter Altar

AtCoder

IDabc174_d

Time2000ms

Memory256MB

Difficulty—

An altar enshrines $N$ stones arranged in a row from left to right. The color of the $i$\-th stone from the left $(1 \leq i \leq N)$ is given to you as a character $c_i$; `R` stands for red and `W` stands for white. You can do the following two kinds of operations any number of times in any order: * Choose two stones (not necessarily adjacent) and swap them. * Choose one stone and change its color (from red to white and vice versa). According to a fortune-teller, a white stone placed to the immediate left of a red stone will bring a disaster. At least how many operations are needed to reach a situation without such a white stone? ## Constraints * $2 \leq N \leq 200000$ * $c_i$ is `R` or `W`. ## Input Input is given from Standard Input in the following format: $N$ $c_{1}c_{2}...c_{N}$ [samples]

Samples

Input #1

4
WWRR

Output #1

2

For example, the two operations below will achieve the objective.

*   Swap the $1$\-st and $3$\-rd stones from the left, resulting in `RWWR`.
*   Change the color of the $4$\-th stone from the left, resulting in `RWWW`.

Input #2

2
RR

Output #2

0

It can be the case that no operation is needed.

Input #3

8
WRWWRWRR

Output #3

API Response (JSON)

{
  "problem": {
    "name": "Alter Altar",
    "description": {
      "content": "An altar enshrines $N$ stones arranged in a row from left to right. The color of the $i$\\-th stone from the left $(1 \\leq i \\leq N)$ is given to you as a character $c_i$; `R` stands for red and `W` st",
      "description_type": "Markdown"
    },
    "platform": "AtCoder",
    "limit": {
      "time_limit": 2000,
      "memory_limit": 262144
    },
    "difficulty": "None",
    "is_remote": true,
    "is_sync": true,
    "sync_url": null,
    "sign": "abc174_d"
  },
  "statements": [
    {
      "statement_type": "Markdown",
      "content": "An altar enshrines $N$ stones arranged in a row from left to right. The color of the $i$\\-th stone from the left $(1 \\leq i \\leq N)$ is given to you as a character $c_i$; `R` stands for red and `W` st...",
      "is_translate": false,
      "language": "English"
    }
  ]
}

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