public static void main ( String [ ] args ) throws IOException {
  Scanner sc = new Scanner ( System . in ) ;
  PrintWriter pw = new PrintWriter ( System . out ) ;
  int n = sc . nextInt ( ) ;
  int [ ] a = new int [ n ] ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) a [ i ] = sc . nextInt ( ) ;
  boolean ok = a [ 0 ] == 0 ;
  for ( int i = 1 ;
  i < n ;
  i ++ ) if ( a [ i ] - a [ i - 1 ] > 1 ) ok = false ;
  if ( ! ok ) {
    System . out . println ( - 1 ) ;
    return ;
  }
  long ans = 0 ;
  for ( int i = 1 ;
  i < n ;
  i ++ ) if ( a [ i ] > a [ i - 1 ] ) ans ++ ;
  else ans += a [ i ] ;
  pw . println ( ans ) ;
  pw . close ( ) ;
}

public static void main ( String [ ] args ) throws IOException { Scanner sc = new Scanner ( System . in ) ; PrintWriter pw = new PrintWriter ( System . out ) ; int n = sc . nextInt ( ) ; int [ ] a = new int [ n ] ; for ( int i = 0 ; i < n ; i ++ ) a [ i ] = sc . nextInt ( ) ; boolean ok = a [ 0 ] == 0 ; for ( int i = 1 ; i < n ; i ++ ) if ( a [ i ] - a [ i - 1 ] > 1 ) ok = false ; if ( ! ok ) { System . out . println ( - 1 ) ; return ; } long ans = 0 ; for ( int i = 1 ; i < n ; i ++ ) if ( a [ i ] > a [ i - 1 ] ) ans ++ ; else ans += a [ i ] ; pw . println ( ans ) ; pw . close ( ) ; }
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