do  {
  operace = prompt("Zadej operaci součet +, rozdíl -, součin *, podíl/", "");
  switch(operace) {
    case "+":
      vysledek = a + b;
      alert (a.toString() + " " + operace  + " " + b.toString() + " = " + vysledek.toString());
      break;
    case "-":
      vysledek = a - b;
      alert (a.toString() + " " + operace  + " " + b.toString() + " = " + vysledek.toString());
      break;
    case "*":
      vysledek = a * b;
      alert (a.toString() + " " + operace  + " " + b.toString() + " = " + vysledek.toString());
      break;
    case "/":
      if (b != 0) {
        vysledek = a / b; 
        alert (a.toString() + " " + operace  + " " + b.toString() + " = " + vysledek.toString()); 
      } else {
        alert ("Nelze dělit nulou."); 
      }      
      break;         
  } 
} while ((operace == "+") || (operace == "-") || (operace == "*") || (operace == "/"));