Period

#include <iostream>
#include <string>

int main()
{
	std::string s;
	std::getline(std::cin, s);

	size_t periodLen = 1;
	for (size_t i = 1; i < s.size(); ++i) {
		size_t j = (periodLen == 1) ? 0 : (i % periodLen);
		if (s[i] != s[j]) periodLen = (i + 1);
	}

	std::cout << s.substr(0, periodLen) << std::endl;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED