public static void main ( String [ ] args ) throws IOException {
  InputReader2 ir = new InputReader2 ( ) ;
  PrintWriter pw = new PrintWriter ( System . out ) ;
  int n = ir . nextInt ( ) ;
  long a = 0L ;
  long b = 0L ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    int tmp = ir . nextInt ( ) ;
    int half = tmp / 2 ;
    if ( i % 2 == 0 ) {
      a += half ;
      b += ( tmp - half ) ;
    }
    else {
      b += half ;
      a += ( tmp - half ) ;
    }
  }
  pw . println ( Math . min ( a , b ) ) ;
  pw . close ( ) ;
}

public static void main ( String [ ] args ) throws IOException { InputReader2 ir = new InputReader2 ( ) ; PrintWriter pw = new PrintWriter ( System . out ) ; int n = ir . nextInt ( ) ; long a = 0L ; long b = 0L ; for ( int i = 0 ; i < n ; i ++ ) { int tmp = ir . nextInt ( ) ; int half = tmp / 2 ; if ( i % 2 == 0 ) { a += half ; b += ( tmp - half ) ; } else { b += half ; a += ( tmp - half ) ; } } pw . println ( Math . min ( a , b ) ) ; pw . close ( ) ; }
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