4 numbers are entered from the keyboard, find the smallest, largest (if, then, else)

program zz1;
var a, b, c, d, max, min: integer;
begin
write (‘enter the first number’);
read (a);
write (‘enter the second number’);
read (b);
write (‘enter the third number’);
read (c);
write (‘enter the fourth number’);
read (d);
if (a> b) and (a> c) and (a> d) then max: = a;
if (b> a) and (b> c) and (b> d) then max: = b;
if (c> b) and (c> a) and (c> d) then max: = c;
if (d> b) and (d> c) and (d> a) then max: = d;

if (a <b) and (a <c) and (a <d) then min: = a;
if (b <a) and (b <c) and (b <d) then min: = b;
if (c <b) and (c <a) and (c <d) then min: = c;
if (d <b) and (d <c) and (d <a) then min: = d;
writeln (‘greatest =’, max);
write (‘smallest =’, min);
end.