#!/usr/koeki/bin/ruby

def factorial(m)
  if m == 1
    1
  else
    m * factorial(m-1)
  end
end

def combination(n,m)
  factorial(n)/factorial(m)/factorial(n-m)

  # factorial(n)/(factorial(m) * factorial(n-m))
  # でもよい
end

all = 7
picked = 2

printf("%dC%dは%dです\n", all, picked, combination(all,picked))