If the first digit is 1 , then make it 0 and add 9 at the end of the string .
if ( a [ 0 ] == 1 and s == s1 ) { ll l = s . size ( ) ; a = " " ; a += '0' ; for ( int i = 0 ; i < l ; i ++ ) a += '9' ; } else { for ( int i = a . size ( ) - 1 ; i >= 0 ; i -- ) {