#include <iostream>
#include <cstdio>
#include <cmath>

using namespace std;

int main(){
    double a, l, x;
    while(cin >> a >> l >> x){
        double ret = 0;
        double d = sqrt(abs(l*l-(a/2)*(a/2)));
        ret += a*d/2;
        double e = sqrt(pow((l+x)/2, 2)-pow((l/2), 2));
        ret += e*l;
        printf("%.9f\n", ret);
    }
}