public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int event_num = sc . nextInt ( ) ;
  int toatl_time = sc . nextInt ( ) ;
  int sum = 0 ;
  int total_jokes = 0 ;
  int arr [ ] = new int [ event_num ] ;
  for ( int i = 0 ;
  i < event_num ;
  i ++ ) {
    arr [ i ] = sc . nextInt ( ) ;
    sum = sum + arr [ i ] ;
  }
  if ( toatl_time < sum + ( event_num - 1 ) * 10 ) {
    System . out . println ( "-1" ) ;
    return ;
  }
  else {
    total_jokes = ( ( ( event_num - 1 ) * 10 ) / 5 ) + ( toatl_time - ( sum + ( event_num - 1 ) * 10 ) ) / 5 ;
    System . out . println ( total_jokes ) ;
  }
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int event_num = sc . nextInt ( ) ; int toatl_time = sc . nextInt ( ) ; int sum = 0 ; int total_jokes = 0 ; int arr [ ] = new int [ event_num ] ; for ( int i = 0 ; i < event_num ; i ++ ) { arr [ i ] = sc . nextInt ( ) ; sum = sum + arr [ i ] ; } if ( toatl_time < sum + ( event_num - 1 ) * 10 ) { System . out . println ( " - 1" ) ; return ; } else { total_jokes = ( ( ( event_num - 1 ) * 10 ) / 5 ) + ( toatl_time - ( sum + ( event_num - 1 ) * 10 ) ) / 5 ; System . out . println ( total_jokes ) ; } }
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