public static void main ( String args [ ] ) {
  int n = sc . nextInt ( ) ;
  int x = sc . nextInt ( ) ;
  int a = 1 ;
  int res = 0 ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    int s = sc . nextInt ( ) ;
    int f = sc . nextInt ( ) ;
    if ( x == 1 ) {
      res = res + f - s + 1 ;
    }
    else {
      while ( a + x <= s ) {
        a = a + x ;
      }
      res = res + f - a + 1 ;
      a = f + 1 ;
    }
  }
  System . out . println ( res ) ;
}

public static void main ( String args [ ] ) { int n = sc . nextInt ( ) ; int x = sc . nextInt ( ) ; int a = 1 ; int res = 0 ; for ( int i = 0 ; i < n ; i ++ ) { int s = sc . nextInt ( ) ; int f = sc . nextInt ( ) ; if ( x == 1 ) { res = res + f - s + 1 ; } else { while ( a + x <= s ) { a = a + x ; } res = res + f - a + 1 ; a = f + 1 ; } } System . out . println ( res ) ; }
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