As it has already been pointed out, the complete solution in Python 3 could be:

import re
m = re.search(r'([a-zA-Z0-9])\1+', input().strip())
print(m.group(1) if m else -1)

My solution has an even shorter regex, but I wrote it only for fun and I'd suggest not to use it to solve such a simple problem:

(\w(?!_))\1+