not able to merge table cells

hello forum,

i am using Indesign 5.5, having problem to merge table cells through scripting.

i want to merge multiple cells in a table.

i have merge first three merge coorectly, but my script not able to merge forth merge.

i am using,

var myDocument=app.activeDocument;

var myTextFrame=myDocument.spreads.item(0).textFrames.item("Spread0TxtFrame0");

var table =myTextFrame.tables.firstItem();

var t = table.rows.length;

var f = table.rows.length - (t-start row count) // **start row count is intiger indicates merge start row

var fn=table.rows.length - (t-end row count)

table.columns[1].cells.merge table.columns[2].cells[fn];      ////* [1]=starting column,,,[2]= end column

when i does pass the value for first three merge it works but when i use for forth merge indxing is change for value of rows.what i am passing into f and fn.

example:-

var t = table.rows.length;

var f = table.rows.length - (t-0);

var fn=table.rows.length - (t-5);

table.columns[0].cells.merge table.columns[0].cells[fn];   // it is for first merge

var t = table.rows.length;

var f = table.rows.length - (t-0);

var fn=table.rows.length - (t-0);

table.columns[2].cells.merge table.columns[3].cells[fn];   // it is for second merge

var t = table.rows.length;

var f = table.rows.length - (t-1);

var fn=table.rows.length - (t-1);

table.columns[1].cells.merge table.columns[2].cells[fn];   // it is for third merge

var t = table.rows.length;

var f = table.rows.length - (t-3);

var fn=table.rows.length - (t-3);   

table.columns[1].cells.merge table.columns[3].cells[fn];   // it is for forth merge

i get

an extra row merges with forth one..

seperately all four merge works but when i fire script with all it merge wrong cells in forth merge.

please correct me where i am wrong in script.

your answer will be apriciated.

Thanks

Ajay