How to add a Google Apps script?

Question

Google Apps Script

Guys, hello!
Help write a script.
I'm not a programmer, but after smoking Google, I managed to do this

function myFunction() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var code1 = ss.getActiveSheet().getRange("A3").getValue();
  var code2 = ss.getActiveSheet().getRange("M4").getValue();
  var value1 = ss.getActiveSheet().getRange("E3").getValue();
  var value2 = ss.getActiveSheet().getRange("N4").getValue();
  
   if (code1==code2){
    var sum = value1 + value2;
     var rr = ss.getActiveSheet().getRange("E3").setValue(sum);
  }
   
 
}

Everything that is written here is done for one cell.
It checks cell M4, if this cell matches A3, then it sums E3 and N4
A3 - this is the product code in the table
M4 - this is the product code that I enter manually
E3 - the amount of goods in the warehouse
N4 - the amount of goods that arrived and I enter it manually .
Actually, as I said above, it all works with one cell.
But I need it to work for each cell separately. For example, I enter the code "2345" it found this cell, for example it will be A45 and after it found it, but E45 contributed the sum of E45 and N4.

Answer 1

Getting data for one cell is sooooooo long. It is better to take all the data at once in the desired range, like this:
And then you can access this data by row number and column number, not forgetting that the countdown starts from zero:
You also need to write down in bulk. As a result, something like this will turn out:

function onOpen() { 
  //Выполняется при открытии
  SpreadsheetApp
  .getUi()
  .createMenu('Меню')
  .addItem('Выполнить','doIt')
  .addToUi();
};

function doIt() {
  try{
    var ss = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet(); //Текущий лист
    
    var data1 = ss.getRange("A3:E" + ss.getMaxRows()).getValues();
    var data2 = ss
    .getRange("m4:n" + ss.getMaxRows())
    .getValues()
    .filter(function(row) // Убираем пустые строки
            {
              return row[0]!="";
            }
           );
    
    for (var row2=0;row2<data2.length;row2++){
      for (var row1=0;row1<data1.length;row1++){
        if (data1[row1][0]===data2[row2][0]){
          data1[row1][4]+=data2[row2][1];
          break; //Если значение найдено, дальше не ищем
        };
      };
    };
    
    ss.getRange("A3:E" + ss.getMaxRows()).setValues(data1); //Вывод данных
    SpreadsheetApp.getActive().toast("Готово!");
  }catch(e){ //Если вдруг ошибка
    Logger.log("Ошибка! " + e);
  };
}

Demo table