public static void main ( String args [ ] ) {
  FastReader in = new FastReader ( ) ;
  PrintWriter so = new PrintWriter ( new BufferedWriter ( new OutputStreamWriter ( System . out ) ) ) ;
  int d = in . nextInt ( ) ;
  int n = in . nextInt ( ) ;
  int a [ ] = new int [ n ] ;
  int ans = 0 ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    a [ i ] = in . nextInt ( ) ;
    if ( i < n - 1 ) ans += d - a [ i ] ;
  }
  so . println ( ans ) ;
  so . flush ( ) ;
}

public static void main ( String args [ ] ) { FastReader in = new FastReader ( ) ; PrintWriter so = new PrintWriter ( new BufferedWriter ( new OutputStreamWriter ( System . out ) ) ) ; int d = in . nextInt ( ) ; int n = in . nextInt ( ) ; int a [ ] = new int [ n ] ; int ans = 0 ; for ( int i = 0 ; i < n ; i ++ ) { a [ i ] = in . nextInt ( ) ; if ( i < n - 1 ) ans += d - a [ i ] ; } so . println ( ans ) ; so . flush ( ) ; }
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