又水一题

#include<iostream>
#include<cstdio>
#include<cmath>
using namespace std;
int main(){
    double a,b,c,d;
    cin>>a>>b>>c;
    d=(a+b+c)/2.0;
    printf("%.2f",sqrt(d*(d-a)*(d-b)*(d-c)));
    return 0;
}