#include <iostream>
#include <iomanip>
#include <sstream>
#include <cstdio>
#include <string>
#include <vector>
#include <algorithm>
#include <complex>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <cassert>
#include <climits>
#include <queue>
#include <set>
#include <map>
#include <valarray>
#include <bitset>
#include <stack>
using namespace std;

#define REP(i,n) for(int i=0;i<(int)n;++i)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)
#define ALL(c) (c).begin(), (c).end()
typedef long long ll;
typedef pair<int,int> pii;
const int INF = 1<<29;
const double PI = acos(-1);
const double EPS = 1e-8;



int main() {
  double a, l, x;
  while(cin>>a>>l>>x) {
    double s = (a+l+l)/2;
    double S = sqrt(s*(s-a)*(s-l)*(s-l));
    x = (l+x)/2;
    s = (l+x+x)/2;
    S += 2*sqrt(s*(s-l)*(s-x)*(s-x));
    printf("%.10f\n", S);
  }
}