#include <bits/stdc++.h>
template<class T> inline bool chmin(T&a, T b){if(a > b){a = b; return true;}else{return false;}}
template<class T> inline bool chmax(T&a, T b){if(a < b){a = b; return true;}else{return false;}}
#define ll long long
#define double long double
#define rep(i,n) for(int i=0;i<(n);i++)
#define REP(i,n) for(int i=1;i<=(n);i++)
#define mod (ll)(1e9+7)
#define inf (ll)(3e18+7)
#define eps (double)(1e-9)
#define pi (double) acos(-1)
#define P pair<ll,ll>
#define PiP pair<int,pair<int,int>>
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
using namespace std;

int main() {
    ll x, y, z;
    double w;
    cin >> x >> y >> z >> w;
    for(ll i = 1; i*i <= x; i++){
        if(x % i)continue;
        if(i >= y && x/i >= z){
            double bp = sqrtl(y*y+x/i*x/i);
            double bq = sqrtl(z*z+i*i);
            if(abs(bp-bq-w) < eps){
                double ans = 0;
                ans += x;
                ans -= (double)(x/i*y)/2.0;
                ans -= (double)(i*z)/2.0;
                ans -= (double)((i-y)*(x/i-z))/2.0;
                cout << fixed << setprecision(15) << ans << endl;
                return 0;
            }
        }
        if(x/i >= y && i >= z){
            double bq = sqrtl(z*z+x/i*x/i);
            double bp = sqrtl(y*y+i*i);
            if(abs(bp-bq-w) < eps){
                double ans = 0;
                ans += x;
                ans -= (double)(i*y)/2.0;
                ans -= (double)(x/i*z)/2.0;
                ans -= (double)((x/i-y)*(i-z))/2.0;
                cout << fixed << setprecision(15) << ans << endl;
                return 0;
            }
        }
    }
}