#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

int main()
{
    double a,b,c,d,x,M,h;
    cin>>a>>b>>c>>d;
    if(a==c)
        cout<<"Imposibil";
    else
    {
        x=(d*d-b*b+(c-a)*(c-a))/((c-a)*2);
        if(d*d-x*x<=0)
            cout<<"Imposibil";
        else
            {h=sqrt(d*d-x*x);
            M=((a+c)*h)/2;
            cout<<fixed<<setprecision(2)<<(int)(M*100)/100.0;}
    }
    return 0;
}