In A1 enter 16
In B1 enter 13
In C1 enter:
=IF(A1*B1>=4000000,"STOP HERE","") and copy down

In A2 enter:
=A1*ROW() and copy down

In B2 enter:
=B1*ROW() and copy down

In A1 thru C6 we see:

16 13
32 26
96 78
384 312
1920 1560
11520 9360 STOP HERE