Переделать программу с использованием case

program correctday;
var days, month, years,flag:integer;
begin
write('vvedite den');
read (days);
write('vvedite mesyac');
read (month);
write('vvedite god');
read (years);
flag:=0;
 
if (month=2) and (years mod 4=0)and (days<=29) then flag:=flag+1;
if (month=2)and(days<=28) then flag:=flag+1;
if ((month=1)and(days<=31))or((month=3)and(days<=31))or((month=5)and(days<=31))
or((month=7)and(days<=31))or((month=8)and(days<=31))or((month=10)and(days<=31))
or((month=12)and(days<=31)) then flag:=flag+1;
if ((month=4)and(days<=30))or((month=6)and(days<=30))or((month=9)and(days<=30))
or((month=11)and(days<=30)) then flag:=flag+1;
if years<0 then flag:=0;
if month<0 then flag:=0;
if days<0 then flag:=0;
if flag<>0 then write('yes')
           else write('not');
end.

program correctday;
uses crt;
var days, month, years,flag:integer;
begin
clrscr;
write('vvedite den:  ');
readln (days);
write('vvedite mesyac: ');
readln (month);
write('vvedite god: ');
readln (years);
flag:=0;
case month of
2:case years mod 4 of
  0:case days of
    1..29:flag:=1;
    end;
  else
    case days of
    1..28:flag:=1;
    end;
  end;
4,6,9,11:case days of
         1..30:flag:=1;
         end;
else
 case days of
 1..31:flag:=1;
 end;
end;
if flag=1 then write('yes')
           else write('not');
readln
end.