CODE:
var a,b,c,d:word;
begin
{$IFNDEF ONLINE_JUDGE}
   assign(input, 'input.txt');
   reset(input);
   assign(output, 'output.txt');
   rewrite(output);
{$ENDIF}
read(a,b,c,d);
while ((c>=(a+b)) and (a<=(c-d))) do begin
a:=a+b;
c:=c-d;
end;
If a<=c then
writeln(c);
If c<a then writeln(a);
{$IFNDEF ONLINE_JUDGE}
   close(input);
   close(output);
{$ENDIF}
end.

Wrong answer    2

for exanmple try test
1000 2 2000 3
answer is 1400
1 2 12 3
6
100 100 150 20
150
2 1 1 1
2
150 50 1000 100
450
What then error?

I see an error-

Like, you are saying- "if a<=c then writeln(c)" I think it is wrong. If a+b<=c then answer may be (a+b).

for example, a = 10, b = 7, c = 20, d = 15

Now, the answer is 17.