public static void main ( String [ ] args ) throws IOException {
  br = new BufferedReader ( new InputStreamReader ( System . in ) ) ;
  st = new StringTokenizer ( br . readLine ( ) ) ;
  int n = nxtInt ( ) ;
  PriorityQueue < Integer > q = new PriorityQueue < > ( Collections . reverseOrder ( ) ) ;
  st = new StringTokenizer ( br . readLine ( ) ) ;
  int lmk = nxtInt ( ) ;
  for ( int i = 1 ;
  i < n ;
  i ++ ) q . add ( nxtInt ( ) ) ;
  int cnt = 0 ;
  while ( lmk <= q . peek ( ) ) {
    lmk ++ ;
    int tmp = q . poll ( ) ;
    tmp -- ;
    q . add ( tmp ) ;
    cnt ++ ;
  }
  pr . println ( cnt ) ;
  pr . flush ( ) ;
  pr . close ( ) ;
}

public static void main ( String [ ] args ) throws IOException { br = new BufferedReader ( new InputStreamReader ( System . in ) ) ; st = new StringTokenizer ( br . readLine ( ) ) ; int n = nxtInt ( ) ; PriorityQueue < Integer > q = new PriorityQueue < > ( Collections . reverseOrder ( ) ) ; st = new StringTokenizer ( br . readLine ( ) ) ; int lmk = nxtInt ( ) ; for ( int i = 1 ; i < n ; i ++ ) q . add ( nxtInt ( ) ) ; int cnt = 0 ; while ( lmk <= q . peek ( ) ) { lmk ++ ; int tmp = q . poll ( ) ; tmp -- ; q . add ( tmp ) ; cnt ++ ; } pr . println ( cnt ) ; pr . flush ( ) ; pr . close ( ) ; }
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