module main;

import std;

void main()
{
	// 入力
	string A, B;
	readln.chomp.formattedRead("%s %s", A, B);
	// 答えの計算と出力
	if (A.length != B.length) {
		writeln(A.length > B.length ? A : B);
		return;
	}
	foreach (i; 0 .. A.length.to!int) {
		if (A[i] == B[i]) continue;
		if (A[i] == '4' && B[i] == '7') {
			writeln(A);
			return;
		}
		if (A[i] == '7' && B[i] == '4') {
			writeln(B);
			return;
		}
		if (A[i] != B[i]) {
			writeln(A[i] > B[i] ? A : B);
			return;
		}
	}
}