function getTotalSum(cell) {
    var sheets = SpreadsheetApp.getActiveSpreadsheet().getSheets();
    var sum = 0;
    for (var i = 0; i < sheets.length ; i++ ) {
        var sheet = sheets[i];
        var val = sheet.getRange(cell).getValue();

        if (typeof(val) == 'number') {
            sum += val;
        }
    }

    return sum;
}

function getTotalSum(startrow, startcol) {
  var ss = SpreadsheetApp.getActive();
  var start = new Date();
  var numSheets = ss.getNumSheets(), sum=0;
  for(var k=1; k<numSheets; k++) {
    var data = ss.getSheets()[k].getDataRange().getValues();
    var value = data[startrow-1][startcol-1];
    if(typeof(value) == 'number') {
      sum += value;
    }
  }
  return sum;
}

// global
var ss = SpreadsheetApp.getActive();

function onOpen() {
  ss.addMenu("Sum", [{name: "Go !!", functionName: "getSum"}]);
}

function getSum() {
  // get user input
  var cell = Browser.inputBox('Enter A1 notation', 'like B21',
    Browser.Buttons.OK_CANCEL);

  // retrieve cell reference from a sheet and get row and col index
  var aCell = ss.getActiveSheet().getRange(cell.toString());
  var row = aCell.getRow()-1, col = aCell.getColumn()-1;

  // get number of sheets and set sum to zero
  var numSheets = ss.getNumSheets(), sum=0;

  // iterate through sheets, starting from second sheet
  for(var k=1; k<numSheets; k++) {
    // bulk load sheet at once
    var data = ss.getSheets()[k].getDataRange().getValues();

    // get value from array
    var value = data[row][col];

    // valid value
    if(typeof(value) == 'number') {
      sum += value;
    }
  }

  // return value to active cell
  ss.getActiveCell().setValue(sum);
}