import java.util.Scanner;

public class N293 {

	public static void main(String[] args)
	{
		Scanner sc = new Scanner(System.in);
		long a=sc.nextLong(),b=sc.nextLong();
		long aa=a,bb=b;
		long ans=-1;
		int arank=(int)Math.log10(a)+1,brank=(int)Math.log10(b)+1;
		if(arank!=brank){}
		else
		{
			for(int i=arank;i>0;i--)
			{
				long ten=(long)Math.pow(10, i-1);
				long abuf=aa/ten,bbuf=bb/ten;
				if((abuf==4&&bbuf==7)||(abuf==7&&bbuf==4))
				{ans=(abuf==4)?a:b;break;}
				if(abuf!=bbuf){break;}
				aa-=abuf*ten;bb-=bbuf*ten;
			}

		}

		if(ans==-1)
		{
			ans=(a>=b)?a:b;
		}
		System.out.println(ans);


	}

}