text: const txtN1 = document.getElementById('txtn1');
  const txtN2 = document.getElementById('txtn2');
  const txtOp = document.getElementById('txtop');
  const res = document.querySelector('div.res');
  
  function verificar() {
      if (txtN1.value == '' || txtN2.value == '') {
          alert('Valor inválido');
      } else {
          calcular(Number(txtN1.value),Number(txtN2.value), txtOp.value);
      }
  };