pub trait ChangeBest {
    fn chmin(&mut self, v: Self) -> bool;
    fn chmax(&mut self, v: Self) -> bool;
}

impl<T> ChangeBest for T
where
    T: PartialOrd,
{
    fn chmin(&mut self, v: T) -> bool {
        if *self > v {
            *self = v;
            true
        } else {
            false
        }
    }

    fn chmax(&mut self, v: T) -> bool {
        if *self < v {
            *self = v;
            true
        } else {
            false
        }
    }
}

#[allow(unused_macros)]
macro_rules! mat {
    ($e:expr; $d:expr) => { vec![$e; $d] };
    ($e:expr; $d:expr $(; $ds:expr)+) => { vec![mat![$e $(; $ds)*]; $d] };
}

#[allow(unused_macros)]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        let mut next = || { iter.next().unwrap() };
        input_inner!{next, $($r)*}
    };
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String{
            bytes
                .by_ref()
                .map(|r|r.unwrap() as char)
                .skip_while(|c|c.is_whitespace())
                .take_while(|c|!c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! input_inner {
    ($next:expr) => {};
    ($next:expr, ) => {};

    ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! read_value {
    ($next:expr, ( $($t:tt),* )) => {
        ( $(read_value!($next, $t)),* )
    };

    ($next:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    };

    ($next:expr, chars) => {
        read_value!($next, String).chars().collect::<Vec<char>>()
    };

    ($next:expr, usize1) => {
        read_value!($next, usize) - 1
    };

    ($next:expr, $t:ty) => {
        $next().parse::<$t>().expect("Parse error")
    };
}

fn main() {
    input! {
        s: String
    };

    let len = s.chars().filter(|c| *c == 'a').count();
    let t = (0..len).map(|_| 'a').collect::<String>();
    println!("{}", t);
}