Replace a substring

You are given three strings $s$, $a$ and $b$. Replace every occurrence of $a$ in $s$ with $b$. The search goes left to right: a found $a$ is replaced and the search continues after the replaced piece — so replacing aa with b in aaaa gives bb.

The first line contains $s$ ($1$ to $1000$ characters, words separated by single spaces), the second line $a$ and the third line $b$ ($1$ to $10$ characters, no spaces).

The changed string.

This is exactly what Python's s.replace(a, b) does. By hand: walk along $s$; if $a$ starts at the current position, append $b$ to the result and jump $|a|$ characters ahead, otherwise append the current character and move one step. ababab with ab → ba gives bababa.