#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("YES");}else{puts("NO");}
#define MAX_N 200005

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

int main() {
    double a, b, c;
    cin >> a >> b >> c;
    double ans = Heron(a,b,c) / 4;
    cout << ans << endl;
    return 0;
}