Tandem

The chef was recently studying about repeated occurrences of substrings in the strings. In particular, he was studying about a string s. He calls a string to be in tandem if it can be represented as concatenation of three equal strings, i.e. it can be represented as AAA where A is a non-empty string. For brevity, he calls such strings tandem strings. For example, "ababab" is a tandem string where