text: //Callback
  var fs=require('fs');
  var sum="";
  
    fs.readFile('files/number1.txt', function doneReading(err, fileContents) {
      sum += fileContents.toString();
      fs.readFile('files/number2.txt', function doneReading(err, fileContents) {
        if (err) {
          console.log("Problem to read file", err);
          //throw new Error("Problem to read a file " + err);
          return;
        }
        sum += fileContents.toString();
        fs.readFile('files/number3.txt', function doneReading(err, fileContents) {
          if (err) {
            console.log("Problem to read file", err);
            return;
          }
          sum += fileContents.toString();
          fs.readFile('files/number4.txt', function doneReading(err, fileContents) {
            if (err) {
              console.log("Problem to read file", err);
              return;
            }
            sum += fileContents.toString();
            fs.readFile('files/number5.txt', function doneReading(err, fileContents) {
              if (err) {
                console.log("Problem to read file", err);
                return;
              }
              sum += fileContents.toString();
              fs.readFile('files/number6.txt', function doneReading(err, fileContents) {
                if (err) {
                  console.log("Problem to read file", err);
                  return;
                }
                  sum += fileContents.toString();
                  console.log("callback do something with it", sum);
              })
            })
          })
        })
      })
    });
  console.log("Sum ",sum);