Нельзя преобразовать тип char к string - PascalABC.NET

//version 1.1
program calculator;
var a, rez: real;
oper, calc: string;
flag_start: boolean;
 
begin
flag_start:=false;
calc:='N;';
rez:=0;
repeat
if (flag_start = false) then
begin
writeln('Are you ready to use calculator? Y/N');
readln(calc);
if (calc <> 'Y') then break;
writeln('Input first number and press Enter');
readln(a);
rez:=a;
writeln('Choose operator: + - * / mod div and press Enter');
readln(oper);
writeln('Input second number and press Enter');
readln(a);
case oper of
'+': rez := rez + a;
'-': rez := rez - a;
'*': rez := rez * a;
'/': rez := rez / a;
'mod': rez := int(round(rez) mod round(a));
'div': rez := int(round(rez) div round(a));
else writeln('You chose wrong operator');
end;
flag_start := true;
end
else
begin
writeln('Choose operator: + - * / mod div and press Enter');
readln(oper);
writeln('Input number and press Enter');
readln(a);
case oper of
'+': rez := rez + a;
'-': rez := rez - a;
'*': rez := rez * a;
'/': rez := rez / a;
'mod': rez := int(round(rez) mod round(a));
'div': rez := int(round(rez) div round(a));
else writeln('You chose wrong operator');
end;
end;
writeln('Will you continue to use a calculator? Y/N');
readln(calc);
until (calc = 'N');
writeln('Result is ', rez:8:2);
readln;
end.