use proconio::{input, marker::Chars};
fn main() {
    input!{
        s: Chars
    }
    let mut cnt = 0;
    let mut ans = 0;
    for i in 0..s.len(){
        if s[i] == '0'{
            ans += cnt/2
        }
        else{
            cnt += 1;
        }
    }
    println!("{}",ans);
}