#include <cstdio>
#include <iostream>
#include <sstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <climits>
#include <cfloat>
using namespace std;

int main()
{
    for(;;){
        double a, l, x;
        if(!(cin >> a >> l >> x))
            return 0;

        double y = sqrt((x * x + 2 * l * x) / 4.0);
        double z = sqrt(l * l - a * a / 4.0);
        double ret = l * y + a * z / 2.0;

        printf("%.10f\n", ret);
    }
}