What value does i have on line 5? And on line 6?
Most bigint implementations do that. My own do that. But it really makes little difference. Wait until it does to complain about it before side-tracking the OP. (As it stands, it appears his bigint is a fixed-size, making order a doubly non-issue.)
Numbers are made with digits, not letters in the alphabet. Also, '25' is made up of two digits, not one.