input: Write a function to evaluate a numerical expression given as a string and return the result.
  def eval_expr(expr):
     return eval(expr)
  assert eval_expr("2+3*4")==14
  assert eval_expr("10-5")==5
  assert eval_expr("2**3")==8
  assert eval_expr("15/3")==5.0
  assert eval_expr("100//3")==33
  assert eval_expr("10%3")==1
output: def eval_expr(expr):
     return eval(expr)