Sock Pairs

Joseph is in the middle of packing for a vacation. He's having a bit of trouble finding all of his socks, though.

Write a function that returns the number of sock pairs he has. A sock pair consists of two of the same letter, such as "AA". The socks are represented as an unordered sequence.

Examples

sock_pairs("AA") ➞ 1

sock_pairs("ABABC") ➞ 2

sock_pairs("CABBACCC") ➞ 4

Notes

• If given an empty string (no socks in the drawer), return 0.
• There can be multiple pairs of the same type of sock, such as two pairs of CC for the last example.