// Algorithms //

Recursion

based on pmi(principle of mathematical induction)
task1: prove a small value is true f(0),f(1)
task2: asssume f(k) is true
task3: prove f(k+1) is true
eg

def fact(n):
  if n==0:
    return 1
  return n * fact(n-1)

n=int(input())
print(fact(n))