char a[99],b[99];f;i;main(){for(scanf("%s%s",a,b);strlen(a)==strlen(b)&&a[i]==b[i];i++);puts(strlen(a)<strlen(b)||(a[i]<b[i]||a[i]=='7'&&b[i]=='4')&&(a[i]!='4'||b[i]!='7')?b:a);}