use itertools::Itertools;
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 {
        Some(x.chars().interleave(y.chars()).collect())
    } else {
        None
    }
}