output_description: Print the number of ways modulo $10^9+7$ in a line.
input_description: $n$ $k$
      
  
  The first line will contain two integers $n$ and $k$.
attempt: N, K = map(int, input().split())
  mod = 10**9 + 7
  
  
  def mod_cmb(n, r, mod=10**9 + 7):
      assert n >= r >= 0
  
      def ex_euclid(x, y):
          a, b = 1, 0
          while y != 0:
              a, b = b, (a - x // y * b)
              x, y = y, x % y
          return a
  
      p = q = 1
      for i in range(n - r + 1, n + 1):
          p *= i
          p %= mod
      for i in range(2, r + 1):
          q *= i
          q %= mod
      p *= ex_euclid(q, mod)
      p %= mod
      return p
  
  
  print(mod_cmb(N + K - 1, N))
  
problem_description: You have $n$ balls and $k$ boxes. You want to put these balls into the boxes.
  
  Find the number of ways to put the balls under the following conditions:
  
    * Each ball is **not** distinguished from the other.
    * Each box is distinguished from the other.
    * Each ball can go into only one box and no one remains outside of the boxes.
    * Each box can contain an arbitrary number of balls (including zero).
  
  Note that you must print this count modulo $10^9+7$.
samples: [{"input": "5 3", "output": "21"}, {"input": "10 5", "output": "1001"}, {"input": "100 100", "output": "703668401"}]