fast way for string comparison

I have a simple question but it makes me confused.

I have two strings, and I want to count how many different characters between the two. The strings are sorted, equal length. Do not split the strings.

For example

input:  abc, bcd
output: 2, because a and d are different characters

input:  abce, bccd
output: 4, because a, c, d and e are different.

I know I can do it in O(N2), but how can I solve it in O(N) for these sorted strings?