#include <iostream>
#include <vector>
#include <algorithm>
#include <utility>
#include <string>
#include <sstream>
#include <cstring>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cctype>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <bitset>

#define rep(i, n) for(int i = 0; i < (n); i++)
#define FOR(i, a, b) for(int i = (a); i < (b); i++)
#define all(v) (v).begin(), (v).end()
#define rev(s) (s).rbegin(), (s).rend()
#define MP make_pair
#define X first
#define Y second

using namespace std;

typedef long long ll;
typedef pair<int, int> P;
typedef vector<int> vi;

double calcs(double a, double b, double c){
	double s = (a+b+c)/2;
	return sqrt(s*(s-a)*(s-b)*(s-c));
}

int main(){
	cout.setf(ios::fixed);
	cout.precision(8);
	double a, l, x;
	while(cin >> a >> l >> x){
		cout << calcs(a, l, l) + calcs(l, (l+x)/2, (l+x)/2)*2 << endl;
	}

	return 0;
}