пример "a arbalbetaa" = "rbalbetaa"
uses crt;
var s:string;
i:byte;
begin
clrscr;
writeln('Введите строку');
readln(s);
for i:=length(s) downto 1 do
if(s[i] in ['a','b'])and not(s[i-1] in ['a','b'])and not(s[i+1] in ['a','b'])
then delete(s,i,1);
write(s);
readln
end.