#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
#define rep(i,m,n) for(int i=m; i<int(n); ++i)
#define repll(i,m,n) for(ll i=m; i<ll(n); ++i)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define leng(a) int(a.size())
#define fi first
#define se second

template<typename T> bool chmin(T& a, T b){ if(a > b){a = b; return true;} return false; }
template<typename T> bool chmax(T& a, T b){ if(a < b){a = b; return true;} return false; }

template<typename T> T gcd(T a, T b){ return a % b ? gcd(b, a % b) : b; }
template<typename T> T lcm(T a, T b){ return a / gcd(a, b) * b; }

char comp(char c, char d){
    if((c == '4' && d == '7') || (c == '7' && d == '4')){
        return c == '4' ? c : d;
    }else{
        return max(c, d);
    }
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    string a, b;
    cin >> a >> b;

    if(leng(a) != leng(b)){
        cout << (leng(a) > leng(b) ? a : b) << endl;
    }else{
        rep(i, 0, leng(a)){
            if(a[i] == b[i]) continue;
            cout << (comp(a[i], b[i]) == a[i] ? a : b) << endl;
            break;
        }
    }
    return 0;
}