use std::io::Read as _;

fn main() {
    let mut s = vec![0; 0x2_000_001];
    let len = std::io::stdin().read(s.as_mut()).unwrap();

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

    for b in 0..len {
        let c = s[b];
        if c == b'0' {
            ans += (b - a) / 2;
            a += 1 + (a + b) % 2;
        }
    }

    println!("{ans}");
}