#include <iostream>
#include <string>
#include <sstream>
#include <algorithm>
#include <vector>
#include <utility>
#include <functional>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <deque>
#include <ctime>
  
using namespace std;
  
#define rep(i,n) REP(i,0,n)
#define REP(i,s,e) for(int i=(s); i<(int)(e); i++)
#define pb push_back
#define mp make_pair
#define all(r) r.begin(),r.end()
#define rall(r) r.rbegin(),r.rend()
#define fi first
#define se second
#define println(X) cout<<X<<endl;
#define DBG(X) cout<<#X<<" : "<<X<<endl;
  
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vii;
typedef vector<ll> vl;
typedef vector<vl> vll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
 
const int INF = 1e9;
const ll MOD = 1e9 + 7;
double EPS = 1e-8;
const double PI = acos(-1);

double helon(double a, double b, double c){
    double ret = 0.0;
    double s = (a+b+c)/2;
    ret = sqrt(s*abs(a-s)*abs(b-s)*abs(c-s));
    return ret;
}

int main(){
    int a, l, x;
    while(cin>>a>>l>>x){
        double s = helon(a, l, l)+2*helon(l, (l+x)/2.0, (l+x)/2.0);//*max({helon(l, (l+x)/2.0, (l+x)/2.0), helon(x, l, l)});
        printf("%.15lf\n", s);
    }
}