Would someone please help me figure out how to get my compiler to return a value of -1 for this function?

#include <iostream>

#include <cmath>

using namespace std;

double TriangleArea( double sideA, double sideB, double sideC )

{

double S, area;

S = (sideA + sideB + sideC) / 2.0;

// Function to calculate the area of a triangle

area = ( S * (S - sideA) * (S - sideB) * (S - sideC) );

if(S > 0)

{

area = sqrt( area );

}

else

{

area = -1;

}

return area;

}

int main()

{

double S, area, sideA, sideB, sideC;

// Enter the sides of the triangle.

cout << "Enter the sides of the triangle: \n";

cin >> sideA >> sideB >> sideC;

// calling back The TriangleArea function

area = TriangleArea( sideA, sideB, sideC );

if(area == -1)

{

cout << "Triangle cannot be formed with given sides" << endl;

}

else

{

cout << "\nArea of triangle = " << TriangleArea( sideA, sideB, sideC ) << endl;

}

return 0;

}