Programming

It looks that I made a mistake somewhere. I recalced it a few times by my own, and my answer and program’s answer are the same, but the site returns error.
There is a log of program working:

Site says:

Un+1 = [ 7 + Un ] + [ n * 15 ]
U0 = -779
You must find U657553

I say:

U0: -779
U1: -772
d: 7
n: 657553
answer = U1 + d * ( n - 1 )
answer: 4602092
Your result is incorrect!
Try again!

Wrong formula was my mistake...