Проверить является ли квадратом по 4 точкам — Pascal(Паскаль)

 Program zadanie3;
Uses crt;
Var x1,y1,x2,y2,x3,y3,x4,y4:integer;
    d1,d2,d3,d4:real;
Begin
Write('Vvedite koordinati x1 i y1: ');
read(x1,y1);
Write('Vvedite koordinati x2 i y2: ');
read(x2,y2);
Write('Vvedite koordinati x3 i y3: ');
read(x3,y3);
Write('Vvedite koordinati x4 i y4: ');
read(x4,y4);
d1:=sqrt(sqr(x1-x2)+sqr(y1-y2));
d2:=sqrt(sqr(x2-x3)+sqr(y2-y3));
d3:=sqrt(sqr(x3-x4)+sqr(y3-y4));
d4:=sqrt(sqr(x4-x1)+sqr(y4-y1));
if (d1=d2)and (d3=d4) and (d1=d3) then write('kvadrat')
else write('Ne yavlyaetsya kvadratom');
end.