public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int n = sc . nextInt ( ) ;
  int arr [ ] = new int [ n ] ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    arr [ i ] = sc . nextInt ( ) ;
  }
  if ( arr [ 0 ] != 0 ) {
    System . out . println ( - 1 ) ;
    return ;
  }
  for ( int i = 1 ;
  i < n ;
  i ++ ) {
    int dif = arr [ i ] - arr [ i - 1 ] ;
    if ( dif > 1 ) {
      System . out . println ( - 1 ) ;
      return ;
    }
  }
  long ans = 0 ;
  for ( int i = n - 1 ;
  i >= 0 ;
  i -- ) {
    ans += arr [ i ] ;
    if ( arr [ i ] > 0 ) {
      while ( i > 0 && arr [ i ] - arr [ i - 1 ] == 1 ) {
        i -- ;
      }
    }
  }
  System . out . println ( ans ) ;
  return ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int n = sc . nextInt ( ) ; int arr [ ] = new int [ n ] ; for ( int i = 0 ; i < n ; i ++ ) { arr [ i ] = sc . nextInt ( ) ; } if ( arr [ 0 ] != 0 ) { System . out . println ( - 1 ) ; return ; } for ( int i = 1 ; i < n ; i ++ ) { int dif = arr [ i ] - arr [ i - 1 ] ; if ( dif > 1 ) { System . out . println ( - 1 ) ; return ; } } long ans = 0 ; for ( int i = n - 1 ; i >= 0 ; i -- ) { ans += arr [ i ] ; if ( arr [ i ] > 0 ) { while ( i > 0 && arr [ i ] - arr [ i - 1 ] == 1 ) { i -- ; } } } System . out . println ( ans ) ; return ; }
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