#include<stdio.h>
#include<string.h>
main(){
    char A[11],B[11];
    scanf("%s %s",A,B);
    if(strlen(A) != strlen(B)){
        printf("%s\n",strlen(A)>strlen(B) ? A:B);
    }else{
        for(int i = 0;i < strlen(A);i++){
            if(A[i] == B[i]){continue;}
            if(A[i] == '4' && B[i] == '7'){
                printf("%s\n",A);

            }else if(A[i] == '7' && B[i] == '4'){
                printf("%s\n",B);
            }else{
                printf("%s\n",A[i]>B[i]?A:B);
            }
            break;
        }
    }
}