#include<bits/stdc++.h>

using namespace std;
using ll = long long;

ll len(ll n){
    ll res = 0;
    while(n > 0){
        res++;
        n /= 10;
    }
    return res;
}

bool comp(ll a,ll b){
    if((a == 4 && b == 7) || (a == 7 && b == 4)){
        return true;
    }else{
        return false;
    }
}

int main(){
    ll a,b;
    cin >> a >> b;
    if(len(a) > len(b)){
        cout << a << endl;
    }else if(len(a) < len(b)){
        cout << b << endl;
    }else{
        ll ans = 0LL,A = a,B = b;
        while(A > 0 && B > 0){
            ll xa = A % 10LL,xb = B % 10LL;
            if(comp(xa,xb)){
                if(xa == 4 && xb == 7) ans = a;
                else if(xa == 7 && xb == 4) ans = b;
            }else{
                if(xa > xb) ans = a;
                else if(xa < xb) ans = b;
            }
            A /= 10LL;
            B /= 10LL;
        }
        cout << ans << endl;
    }
    return 0;
}