String Periods

Have the function StringPeriods(str) take the str parameter being passed and determine if there is some substring K that can be repeated N > 1 times to produce the input string exactly as it appears. Your program should return the longest substring K, and if there is none it should return the string -1.

For example: if str is "abcababcababcab" then your program should return abcab because that is the longest substring that is repeated 3 times to create the final string. Another example: if str is "abababababab" then your program should return ababab because it is the longest substring. If the input string contains only a singleExamples

Input: "abcxabc"
Output: -1

Input: "affedaaffed"

Output: -1 character, your program should return the string -1.