public static void main ( String [ ] args ) {
  FastReader In = new FastReader ( ) ;
  int n = In . nextInt ( ) ;
  PriorityQueue < Integer > pq = new PriorityQueue < > ( Collections . reverseOrder ( ) ) ;
  int l = In . nextInt ( ) ;
  for ( int i = 0 ;
  i < n - 1 ;
  i ++ ) {
    pq . add ( In . nextInt ( ) ) ;
  }
  int cont = l ;
  while ( l <= pq . peek ( ) ) {
    l ++ ;
    pq . add ( pq . poll ( ) - 1 ) ;
  }
  System . out . println ( l - cont ) ;
}

public static void main ( String [ ] args ) { FastReader In = new FastReader ( ) ; int n = In . nextInt ( ) ; PriorityQueue < Integer > pq = new PriorityQueue < > ( Collections . reverseOrder ( ) ) ; int l = In . nextInt ( ) ; for ( int i = 0 ; i < n - 1 ; i ++ ) { pq . add ( In . nextInt ( ) ) ; } int cont = l ; while ( l <= pq . peek ( ) ) { l ++ ; pq . add ( pq . poll ( ) - 1 ) ; } System . out . println ( l - cont ) ; }
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