program f;
var
q0, q1, p0, p1: real;
p, q: real;
begin
write('q0: ');
readln(q0);
write('q1: ');
readln(q1);
write('p0: ');
readln(p0);
write('p1: ');
readln(p1);
p := (p1 + p0) / (p1 - p0);
q := (q1 - q0) / (q1 + q0);
writeln('E= ', q * p:0:2);
readln;
end.