#include<bits/stdc++.h>
using namespace std;
int main(){
    float a,b,c;
    float s;
    cin>>a>>b>>c;
    float p=(a+b+c)/2;
    s=sqrt(p*(p-a)*(p-b)*(p-c));
    printf("%0.1f\n",s);
    return 0;
}