I don't understand pcf

#include <iostream>
#include <cmath>
#include <iomanip>

int main () {
    int x1, y1, x2, y2, x3, y3, s1, s2, s3;
    std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    s1 = (x1 - x2) * (x1 - x3) + (y1 - y2) * (y1 - y3);
    s2 = (x1 - x3) * (x3 - x2) + (y1 - y3) * (y3 - y2);
    s3 = (x3 - x2) * (x1 - x2) + (y3 - y2) * (y1 - y2);
    if (s1 + s2 + s3 == 0){
        std::cout << 0;
    } else {
        if (s1 + s2 == 0||s1 + s3 == 0||s2 + s3 == 0) {
            std::cout << 1;
        } else {
            if (x3 - x1 + y2 - y1 == (y3 - y1) * (x2 - x1)) {
                std::cout << 2;
            } else {
                if (s1 * s2 * s3 == 0) {
                    std::cout << 4;
                } else {
                    if (s1 * s2 * s3 < 0) {
                        std::cout << 5;
                    } else {
                        std::cout << 3;
                    }
                }
            }
        }
    }
    return 0;
}