Найти разницу площадей треугольников - C (СИ)

// ConsoleApplication2.cpp: определяет точку входа для консольного приложения.
//
 
#include "stdafx.h"

int main()
{
    float x1, x2, x3, y1, y2, y3, s;
    printf(" Enter the coordinates of the points A, B, C:\n ");
    scanf_s("%f %f %f %f %f %f", &x1, &y1, &x2, &y2, &x3, &y3);
    float x4, y4, s1, s2, s3, ss, srez;
    printf(" Enter the coordinates of the points A1:\n ");
    scanf_s("%f", &x4, &y4);
    s = (x1*(y2 - y3) + x2*(y3 - y1) + x3*(y1 - y2)) / 2;
    if (s <= 0)
        printf("Triangle  ABC does not exist \n");
    else
    printf(" Enter the coordinates of the points A1:\n ");
    scanf_s("%f %f", &x4, &y4);
    s1 = (x4*(y2 - y3) + x2*(y3 - y4) + x3*(y4 - y2)) / 2;
    s2 = (x1*(y2 - y4) + x2*(y4 - y1) + x4*(y1 - y2)) / 2;
    s3 = (x1*(y4 - y3) + x2*(y3 - y1) + x3*(y1 - y4)) / 2;
    ss = s1 + s2 + s3;
 
    if (s1 = ss) {
        srez = s - s1;
        printf("S= ", srez);
    }
    else
        printf(" Point A1 does not belong to the triangle ABC \n");
    getchar();
    return 0;
}