#include <bits/stdc++.h>
using namespace std;

#define For(i, a, b) for(int i = (a); i < (b); i++)
#define rep(i, n) For(i, 0, n)
#define rFor(i, a, b) for(int i = (a); i >= (b); i--)
#define ALL(v) (v).begin(), (v).end()
#define rALL(v) (v).rbegin(), (v).rend()

using lint = long long;
using ld = long double;

int INF = 2000000000;
lint LINF = 1000000000000000000;

struct SetupIo {
    SetupIo() {
        ios::sync_with_stdio(false);
        cin.tie(nullptr);
        cout << fixed << setprecision(15);
    }
} setupio;

int main() {
    string x, y;
    cin >> x >> y;
    int n = x.size(), m = y.size();
    string ans = "";
    if (n - m == 0 || n - m == 1) {
        int i = 0;
        while (i < n || i < m) {
            if (i < n) { ans += x[i]; }
            if (i < m) { ans += y[i]; }
            i++;
        }
    } else {
        ans = "?";
    }
    cout << ans << "\n";
}