new to programing and need advice on making a simple pascal calculator

Question

(sorry for bad english and code writing) recently ive been interested in programing because of the subject in my school, my teacher wanted us to learn pascal, so ive been learning pascal for a couple days and interested to make a simple calculator, i want to make a logic so when i put a wrong input variable type the program will say "wrong input' and either loops or end the program

and heres my code

program SimpleCalculator;
uses 
  crt;
var
  numA, numB, result : real;
  op, prog : char;
begin
  repeat;
  clrscr; 
  TextColor(15);

  write('enter 1st number: ');
  readln(numA);
  //i want to check if numA is a number, if not the program wont continue or loops back
  write('enter operation: ');
  readln(op);
  write('enter 2nd number: ');
  readln(numB);
  //i want to check if numB is a number, if not the program wont continue or loops back

  if (op = '+') then
    result := numA + numB;

  write('result ', numA : 2 : 1, ' + ', numB : 2 : 1, ' = ', result : 2 : 1);
end

else if (op='-') then
begin
  result := numA - numB;
  write('result ', numA : 2 : 1, ' - ', numB : 2 : 1, ' = ', result : 2 : 1);
end

else if (op = '*') then
begin
  result := numA * numB;
  write('result ', numA : 2 : 1, ' * ', numB : 2 : 1, ' = ', result : 2 : 1);
end

else if (op = '/') then
  if (numA = 0) then
  begin
    write('cannot devide by 0');
    writeln;
    writeln('press (enter) to go back');
    write  ('input (e/E) to exit : ');
    readln(prog);
  end
  else if (numB = 0) then
  begin
    write('cannot devide by 0');
    writeln;
    readln(prog);
  end
  else
  begin
    result := numA / numB;
    write('result ', numA : 2 : 1, ' / ', numB : 2 : 1, ' = ', result : 2 : 1);
  end

  else
    writeln(op,' is not an operator!');

readln;

writeln('press (enter) to go back');
write  ('input (e/E) to exit : ');
readln(prog);
until (prog= 'E') or (prog= 'e');

end.