C++ :: Calculating Area Of A Triangle Using Heron Formula

I need to calculate the area of a triangle using heron's formula. I wrote the code below and i need to modify it so that a user is required to keep entering the three side until he/she decides to stop. If the three sides entered make an invalid triangle, the user is required to re-enter the values until valid triangle is formed. Then the area is displayed.

#include <iostream>
#include <cmath>
using namespace std;
int main(){
double a,b,c=0;
double s,A=0;

[Code] .....

the screen output should like this:

Enter three sides of a triangle: 0 1 2 Error!
Re-enter three sides of a triangle: -1 1 2 Error!
Re-enter three sides of a triangle: 3 4 5 => 6

Continue (y/n)? y

Enter three sides of a triangle: 1 1 2 Error!
Re-enter three sides of a triangle: 6 8 10 => 24

Continue (y/n)? n

Done!

Please note that Continue (y/n)? only displayed after a valid triangle is formed. Otherwise, the user needs to re-enter sides until it's valid.