Mad scientist

Farmer John arranged $n$ of his cows, each of which belongs to one of two breeds: Holsteins or Guernseys. He recorded this arrangement as a string of $n$ characters, each of which is either $H$ or $G$, respectively. Unfortunately, when the cows arrived at the farm and John lined them up again, the resulting string turned out to be different from the original one.

Let us denote these two strings by $a$ and $b$, where $a$ is the original string that Farmer John wanted to see, and $b$ is the string obtained after the cows arrived. Farmer John asked his cousin Ben for help.

After several months of work, Ben created an amazing machine called MCBF-3000, which can select any substring and replace all $G$ characters in it with $H$, and all $H$ characters with $G$. Now Farmer John wants to know the minimum number of applications of this machine required to transform string $b$ into string $a$. Help Farmer John solve this problem.