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