Graph 1: d->b has weight 1 (cycle b->c->d->b weighs -2+3+1 = 2)
pass      d/pi s  d/pi a  d/pi b  d/pi c  d/pi d
init         0/-   inf/-   inf/-   inf/-   inf/-
1 (3)        0/-     3/s     6/s     8/a   inf/-
2 (2)        0/-     3/s     6/s     4/b     7/c
3 (0)        0/-     3/s     6/s     4/b     7/c
4 (0)        0/-     3/s     6/s     4/b     7/c
check: no edge relaxes  =>  return TRUE (no negative cycle reachable)

Graph 2: d->b has weight -2 (cycle b->c->d->b weighs -2+3-2 = -1)
pass      d/pi s  d/pi a  d/pi b  d/pi c  d/pi d
init         0/-   inf/-   inf/-   inf/-   inf/-
1 (3)        0/-     3/s     6/s     8/a   inf/-
2 (3)        0/-     3/s     5/d     4/b     7/c
3 (3)        0/-     3/s     4/d     3/b     6/c
4 (3)        0/-     3/s     3/d     2/b     5/c
check: edge b->c still relaxes (3 + (-2) = 1 < 2)  =>  return FALSE
negative cycle: b c d b
its weight: -1
