input: Write a function to evaluate a given string containing a numerical expression and return the result.
  assert eval_expr("2+3*4")==14
  assert eval_expr("(2+3)*4")==20
  assert eval_expr("5/2")==2.5
  assert eval_expr("10-4")==6
  assert eval_expr("7*3")==21
output: def eval_expr(expression):
      return eval(expression)