#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  string x, y;
  cin >> x >> y;
  if (x.size() == y.size() || x.size() == y.size() + 1) {
    string s = "";
    rep(i, y.size()) s += x[i], s += y[i];
    if (x.size() == y.size() + 1) s += x.back();
    cout << s << endl;
  }
  else {
    cout << "?" << endl;
  }
  return 0;
}