public static void main ( String args [ ] ) {
  Scanner in = new Scanner ( System . in ) ;
  int n = in . nextInt ( ) ;
  int s = in . nextInt ( ) ;
  int a [ ] = new int [ s + 1 ] ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    int x = in . nextInt ( ) ;
    int y = in . nextInt ( ) ;
    a [ x ] = Math . max ( a [ x ] , y ) ;
  }
  int ans = 0 ;
  while ( s > 0 ) {
    if ( a [ s ] - ans >= 0 ) ans = ans + a [ s ] - ans ;
    s -- ;
    ans ++ ;
  }
  System . out . println ( ans ) ;
}

public static void main ( String args [ ] ) { Scanner in = new Scanner ( System . in ) ; int n = in . nextInt ( ) ; int s = in . nextInt ( ) ; int a [ ] = new int [ s + 1 ] ; for ( int i = 0 ; i < n ; i ++ ) { int x = in . nextInt ( ) ; int y = in . nextInt ( ) ; a [ x ] = Math . max ( a [ x ] , y ) ; } int ans = 0 ; while ( s > 0 ) { if ( a [ s ] - ans >= 0 ) ans = ans + a [ s ] - ans ; s -- ; ans ++ ; } System . out . println ( ans ) ; }
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