4 of 6 added 6 characters in body

><>, 46 43 37 + 4 for  -s= = 41 bytes

0&l?!v:3%?\&:n1+$o!
f0.  >&n; >l&:@+n

This is an implementation of xnor's algorithm in ><>.

It takes the input string on the stack (-s flag with the standard interpreter).

You can try it out on the online interpreter.