S1: Префикс-функция

#include <iostream>
#include <fstream>
#include <vector>
#include <string>

using namespace std;


int main() {
    ifstream fin("input.txt");
    ofstream fout("output.txt");

    string str;
    getline(fin, str);

    vector<unsigned long long int> s(str.length(), 0);
    unsigned long long int b, count = 0;
    for (int i = 1; i < str.length(); i++) {
        b = s[i - 1];
        while (b > 0 and str[i] != str[b]) {
            b = s[b - 1];
        }
        if (str[i] == str[b])
            ++b;
        s[i] = b;

        count += b;
    }

    fout << count;
}