use proconio::{input, marker::Chars};

fn main() {
    input! {
        mut s: Chars,
    }

    s.reverse();
    let mut stack = vec![];
    let mut ans = 0;

    while let Some(ch) = s.pop() {
        stack.push(ch);
        if stack.len() < 3 {
            continue;
        }

        if &stack[stack.len() - 3..] == &['1', '1', '0'] {
            let a = stack.pop().unwrap();
            let b = stack.pop().unwrap();
            let c = stack.pop().unwrap();
            s.push(c);
            s.push(b);
            s.push(a);
            ans += 1;
        }
    }

    println!("{}", ans);
}