use proconio::input;

fn main() {
    input! {
        x: String,
        y: String,
    }

    match solve(&x, &y) {
        Some(ans) => println!("{}", ans),
        None => println!("?"),
    }
}

fn solve(x: &str, y: &str) -> Option<String> {
    if x.len() != y.len() && x.len() != y.len() + 1 {
        return None;
    }

    let mut alternate: String = x
        .chars()
        .zip(y.chars())
        .flat_map(|(c1, c2)| [c1, c2])
        .collect();
    if x.len() > y.len() {
        alternate.push(x.chars().last().unwrap());
    }
    Some(alternate)
}