Hi Alexander, thanks for raising the question. The current formula in the article is in fact correct i.e. N = S + D + C. The best way to visualize this is to use examples.

[Example 1]

Ground Truth: abc123 (N=6)

OCR Output: abc1234

S = 0

D = 0

C = 6

I = 1

Using S+D+C gives us the right answer (which is 6), whereas S+I+C gives us 7 (incorrect)

[Example 2]

Ground Truth: abc123 (N=6)

OCR Output: abc12

S = 0

D = 1

C = 5

I = 0

Using S+D+C once again gives us the right answer (i.e. 6), whereas S+I+C gives us 5 (incorrect)