#include<algorithm>
#include<iostream>
#include<cmath>
#include<cstdio>
#include<stack>
#include<vector>
using namespace std;
stack<int> st;
vector<int> v;
double a,b,c,st1,st2,s,h1,h2,all,h3,h4,h5;
int main()
{
    long long k;
    cin>>a>>b>>c;
    all=a+b+c;
    st1=sqrt(a*a+a*a);
    h1=sqrt(st1*st1+st1*st1);
    h2=all-h1;
    if(h2<0) {cout<<0; return 0;}
    st2=st1+sqrt(h2*h2+h2*h2);
    
    h3=st2-st1;
    h4=sqrt(h3*h3+st1*st1);
    h5=a+b;
//    if(sqrt(st1*st1+st1*st1)!=st2) {cout<<0; return 0;} 
    s=st1*st2;
    k=s*10000;

    if(k%10>=5) k=k/10+1;
    else k=k/10;

    while(k!=0)
    {
        v.push_back(k%10); 
        k/=10;
    }
    int i=0;
    for(i=v.size()-1;i>=3;i--)
    {
        cout<<v[i];
    }
    cout<<".";
    for(i=4-1;i>=0;i--)
    {
        cout<<v[i];
    }
    cout<<endl;
}
