Step 1: Divide the number into separate digits:
2906 => 2 9 0 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 9 => 7
9 to 0 => 9
0 to 6 => 6
6 to 2 => 4
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
7 => N
9 => C
6 => N
4 => N
Step 4: Combine the filters to a string:
N C N N = NCNN