public static void main ( String [ ] args ) throws IOException {
  FastScanner fs = new FastScanner ( ) ;
  PrintWriter out = new PrintWriter ( System . out ) ;
  int tt = 1 ;
  while ( tt -- > 0 ) {
    int n = fs . nextInt ( ) ;
    long num1 = 0 , num2 = 0 ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) {
      int a = fs . nextInt ( ) ;
      int b = ( i & 1 ) ;
      num1 += ( a + 1 - b ) / 2 ;
      num2 += ( a + b ) / 2 ;
    }
    out . println ( Math . min ( num1 , num2 ) ) ;
  }
  out . close ( ) ;
}

public static void main ( String [ ] args ) throws IOException { FastScanner fs = new FastScanner ( ) ; PrintWriter out = new PrintWriter ( System . out ) ; int tt = 1 ; while ( tt -- > 0 ) { int n = fs . nextInt ( ) ; long num1 = 0 , num2 = 0 ; for ( int i = 0 ; i < n ; i ++ ) { int a = fs . nextInt ( ) ; int b = ( i & 1 ) ; num1 += ( a + 1 - b ) / 2 ; num2 += ( a + b ) / 2 ; } out . println ( Math . min ( num1 , num2 ) ) ; } out . close ( ) ; }
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