Ошибка -nan(ind) и nan

#include "stdafx.h"
#include <fstream>
#include <math.h>
#include <iomanip>
using namespace std;
double THE_LENGTH_OF_A_SIDE(long, long, long, long);//функция находит длину стороны треугольника
int main()
{
    long x1, x2, x3, y1, y2, y3;
    double s1, s2, s3, p;
    ofstream outputFile("OUTPUT.txt");
    ifstream inputFile("INPUT.txt");
    inputFile >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    s1 = THE_LENGTH_OF_A_SIDE(x1, y1, x2, y2);
    s2 = THE_LENGTH_OF_A_SIDE(x2, y2, x3, y3);
    s3 = THE_LENGTH_OF_A_SIDE(x1, y1, x3, y3);
    outputFile << s1 << " " << s2 << " " << s3;
    p = 1.0  * (s1 + s2 + s3) / 2;
    outputFile << fixed << setprecision(1) << sqrt(abs(p*(p - s1)*(p - s2)*(p - s3)));//формула Герона
    outputFile.close();
    inputFile.close();
}
double THE_LENGTH_OF_A_SIDE(long x1, long y1, long x2, long y2)
{
    double length;
    if (x1 == x2)
    {
        length = 1.0 * abs(y1 - y2);
    }
    else if (y1 == y2)
    {
        length = 1.0 *abs(x1 - x2);
    }
    else
    {
        length = 1.0 * sqrt(abs((x1 - x2)*(x1 - x2)) + abs((y1 - y2)*(y1 - y2)));
    }
    return length;
}

#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
 
int main()
{
    double x1, x2, x3, y1, y2, y3, a, b, c, p, S; 
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
    a = hypot(x1-x2, y1-y2);
    b = hypot(x1-x3, y1-y3);
    c = hypot(x2-x3, y2-y3);
    p = (a+b+c)/2;
    S=sqrt(p*(p - a)*(p - b)*(p - c));
    cout << setprecision(1) << fixed << S <<endl;
    system("pause"); 
    return 0;
}