text: function ianNotations(input) {
      let arr = [];
      let symbols = ["+", "-", "*", "/"];
  
      for (let element of input) {
          if (!symbols.find(n => n === element)) {
              arr.push(Number(element));
              continue;
          }
          if (arr.length < 2) {
              return "Error: not enough operands!";
          }
  
          let num2 = arr.pop();
          let num1 = arr.pop();
          let sum = calculate(num1, num2, element);
  
          arr.push(sum);
  
      }
      if (arr.length > 1) {
         return "Error: too many operands!";
      } else {
          return arr.join("");
      }
  
      function calculate(num1, num2,element) {
          return element === "*" ?
              num1 * num2 : element === "/" ?
                  num1 / num2 : element === "+" ?
                      num1 + num2 : num1 - num2;
      }
  }
  
  console.log(ianNotations([20, 3, 4, '*', '-']));
  
  