use proconio::input;
use proconio::marker::Bytes;

fn main() {
    input! {
        s: Bytes,
    }
    let mut start = 0;
    let mut ans = 0_usize;
    for (i, &c) in s.iter().enumerate() {
        if c == b'0' {
            let next = start + (start + i) % 2;
            ans += (i - next) / 2;
            start = next + 1;
        }
    }
    println!("{ans}");
}