#include <iostream>
#include <cmath>

using namespace std;

const float eps = 1e-20f; //10^-20

int Validate(float a, float b, float c, float d)
{
    //if at least one side < 0.0 -> not trapezoid!
    if(!(a > 0.0f && b > 0.0f && c > 0.0f && d > 0.0f))
        return 0;

    //parallellogram isn't trapezoid! area can not be found!
    if(fabs(a - b) < eps)
        return 0;

    //additional check to distinguish quadrangle from trapezoid
    float ab  =  a - b;
    float arg = (d * d - c * c - ab * ab) / (2.0f * c * ab);

    return fabs(arg) < 1.0f;
}

float Area(float a, float b, float c, float d)
{
    if(!Validate(a, b, c, d))
        return -1.0f;

    //trapezoid area through four sides

    float ab = a - b;
    float t  = 0.5f * (ab * ab + c * c - d * d) / ab;

    return (a + b) * 0.5f * sqrtf(c * c - t * t);
}

int main()
{
    float a, b, c, d;

    cout << "Base 1 = ";
    cin  >> a;

    cout << "Base 2 = ";
    cin  >> b;

    cout << "Side 1 = ";
    cin  >> c;

    cout << "Side 2 = ";
    cin  >> d;

    float s = Area(a, b, c, d);

    if(s < 0.0f)
        cout << "Not trapezoid!" << endl;
    else
        cout << "Area = " << s << endl;

    return 0;
}