正确题解，直接开写！

#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;
double S (double a, double b, double c){
    double p = (a + b + c)/2;
    p = sqrt(p * (p - a) * (p - b) * (p - c));
    return p;
}
int main ()
{
    double a, b, c;
    cin >> a >> b >> c;
    double p = S (a, b, c);
    printf("%.2f", p);
    return 0;
}