strlen(b
+3[r]-1[s]) :
first: 3[r] is same as r[3] i.e., 'E'
and 1[s] is same as s[1] i.e., 'A'
3[r] - 1[s] = 'E' - 'A' = 4
now, b + 4 refers to 4 th index in b i.e, 'C' (starting position)
strlen(b+4) = strlen("CSIT2019") = 8
Thus, it will return 8.
correct me if i am wrong.