#include<iostream>
#include<string>
#include<algorithm>
#include<map>
#include<vector>
#include<cmath>
#include<cstdio>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define pb(in,tmp) in.push_back(tmp)
#define all(in) in.begin(),in.end()
#define PI acos(-1)
using namespace std;
int main(){

  int a,b,c;
  while(cin>>a>>b>>c){
    double sum=0;
    sum+=a/2.0*sqrt(b*b-a*a/4.0);
    sum+=(double)b*sqrt((b+c)*(b+c)/4.0-b*b/4.0);
    
    
    printf("%.20f\n",sum);
  }

}