this.getField("F1t").value is not right

Dear everyone,
My PDF has 2 text field.
i create the javascript to F1 as below:

this.getField("F2").value = this.getField("F1").value;

But result is not right.

i type "123213123123123123123123123" in F1,but F2's value is "1.2321312312312312e+26"

then
app.alert(this.getField("F1").value),that is "1.2321312312312312e+26";

i can not find eny to fix it.can you help me!?
thank you.