var
  x1, y1, x2, y2, x0, y0, x, y, l, l1, l2, d, k, k1: double;

begin
  read(x1, y1, x2, y2, x0, y0, l);
  k:= (y2 - y1) / (x2 - x1 + 1e-8);
  k1:= - 1 / (k + 1e-8);
  x:= (k * x1 - y1 - k1 * x0 + y0) / (k - k1 + 1e-8);
  y:= k * (x - x1) + y1;
  l1:= sqrt(sqr(x0 - x1) + sqr(y0 - y1));
  l2:= sqrt(sqr(x0 - x2) + sqr(y0 - y2));
  if (abs(x1 - x) < 1e-8) or (abs(x2 - x) < 1e-8) or ((x1 - x) * (x2 - x) < 0)
    then d:=sqrt(sqr(x0 - x) + sqr(y0 - y))
    else d:=1e12;

  if l1 < d then d:= l1;
  if l2 < d then d:= l2;
  if d > l
    then writeln(d - l:0:2)
    else writeln('0.00');

  d:= l1;
  if d < l2 then d:= l2;
  if d > l
    then writeln(d - l:0:2)
    else writeln('0.00');
end.


Can somebody give me some special case?

Did you treat the cases in which the triangle is obtuse and acute?

I uses liner equation

you used linear equation but that's wrong. You have to find the distance between a point and a segment,not between a point and a line. Distance between a point and a line is shorter sometimes.