output_description: Print an integer representing the minimum number of operations needed.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
      c_{1}c_{2}...c_{N}
attempt: n = int(input())
  rh_list = list(input())
  # Rの個数
  r_n = rh_list.count("R")
  # Rの個数までのlist
  R_list = []
  for i in range(r_n):
      R_list.append(rh_list[i])
  # listのｒ個数
  R_r_n = R_list.count("R")
  print(r_n - R_r_n)
  
problem_description: Statement
  
  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?
samples: [{"input": "4\n    WWRR", "output": "2\n    \n\nFor example, the two operations below will achieve the objective.\n\n  * Swap the 1-st and 3-rd stones from the left, resulting in `RWWR`.\n  * Change the color of the 4-th stone from the left, resulting in `RWWW`.\n\n* * *"}, {"input": "2\n    RR", "output": "0\n    \n\nIt can be the case that no operation is needed.\n\n* * *"}, {"input": "8\n    WRWWRWRR", "output": "3"}]