[2902] KMP는 왜 KMP일까?

#include <iostream>
#include <string>
using namespace std;
int main()
{
	string s1, s2;
	cin >> s1;
	s2 = s1[0];
	for (int i = 1; i < s1.size(); i++)
		if (s1[i] == '-')
			s2 += s1[i + 1];
	cout << s2;
}