#include <bits/stdc++.h>

using namespace std;

double area(int x1, int y1, int x2, int y2, int x3, int y3) {

return fabs(1.*(x2 — x1)*(y3 — y1)-(1.*(x3 — x1)*(y2 — y1)))/2.;

}

main() {

int x1, y1, x2, y2, x3, y3, a, b, c;

cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;

cout << fixed << area(x1, y1, x2, y2, x3, y3);

}

In this code I don't understand what does 1. mean.

Thank you for your advice!