#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

void solve() {
  string x, y;
  cin >> x >> y;
  ll d = x.size() - y.size();
  string ans = "?";
  if (0 <= d && d <= 1) {
    ans = "";
    rep(i, y.size()) {
      ans.push_back(x[i]);
      ans.push_back(y[i]);
    }
    if (d == 1) {
      ans.push_back(x.back());
    }
  }
  cout << ans << '\n';
}

int main() {
  std::cin.tie(nullptr);
  std::ios_base::sync_with_stdio(false);
  int T = 1;
  for (int t = 0; t < T; t++) {
    solve();
  }
  return 0;
}