macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

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

macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };
    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };
    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}

fn main() {
    input! {
        s: chars
    }
    let mut s = s;
    loop {
        let mut end = true;
        let n = s.len();
        let mut i = 0;
        while i < n - 1 {
            if s[i] == '.' {
                i += 1;
                continue;
            }
            let l = s[i];
            let mut j = i + 1;
            while j < n {
                if s[j] == '.' {
                    j += 1;
                    continue;
                }
                break;
            }
            let r = s[j];
            if l != r {
                s[i] = '.';
                s[j] = '.';
                i += 2;
                end = false;
            } else {
                i += 1;
            }
        }
        if end {
            break;
        }
    }
    println!("{}", s.into_iter().filter(|x| *x == '.').count());
}
