input: Write a function that takes a string containing a numerical expression and returns the result.
  
  def eval_expr(expr):
     return eval(expr)
  
  assert eval_expr("2+3")==5
  assert eval_expr("10-7")==3
  assert eval_expr("4*5")==20
  assert eval_expr("15/3")==5
  assert eval_expr("12+3*5")==27
output: def eval_expr(expr):
     return eval(expr)