fn main() {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).unwrap();

    let mut ans = 0;
    let mut a = 0;

    for (b, &c) in s.as_bytes().iter().enumerate() {
        if c == b'0' {
            ans += (b - a) / 2;
            a += 1 + (a + b) % 2;
        }
    }

    println!("{ans}");
}