Как сделать концовку через case - Turbo Pascal

program CalculatoR;
var a,b,c: integer;
 
Procedure add(a:integer;b:integer);
          begin
               writeln('Sum a+b= ',a+b);
          end;
 
Procedure minus(a:integer;b:integer);
          begin
               writeln('Otnayt a-b= ',a-b);
          end;
 
Procedure mnoz(a:integer;b:integer);
          begin
               writeln('Ymnoz a*b= ',a*b);
          end;
 
Procedure Deleum(a:integer;b:integer);
          begin
               writeln('Del a/b= ',a div b);
          end;
begin
     writeln('                  ***Calculator***');
     writeln('Viberete deystvie 1-Dodat 2-Otnayt 3-Ymnoz 4-Delit');
     readln(c);
 
     if c=1 then
     begin
     writeln('Vvedite dva chisla');
     readln(a,b);
     add(a,b);
     end;
 
      if c=2 then
     begin
     writeln('Vvedite dva chisla');
     readln(a,b);
     minus(a,b);
     end;
 
      if c=3 then
     begin
     writeln('Vvedite dva chisla');
     readln(a,b);
     mnoz(a,b);
     end;
 
      if c=4 then
     begin
     writeln('Vvedite dva chisla');
     readln(a,b);
     Deleum(a,b);
     end;
 
     if c<1 then
        writeln('Error!');
     if c>4 then
        writeln('Error!');

 readln;
end.