module Main where

import Data.List
import Control.Monad
import Debug.Trace
import qualified Data.ByteString.Char8 as BC
import Data.Maybe (fromJust)


readTuple2 :: IO (Int, Int)
readTuple2 = do
    [a, b] <- map (fst . fromJust . BC.readInt) . BC.words <$> BC.getLine
    return (a, b)

readTuple3 :: IO (Int, Int, Int)
readTuple3 = do
    [a, b, c] <- map (fst . fromJust . BC.readInt) . BC.words <$> BC.getLine
    return (a, b, c)

readInt :: IO Int
readInt = fst . fromJust . BC.readInt <$> BC.getLine

readInts :: IO [Int]
readInts = map (fst . fromJust . BC.readInt) . BC.words <$> BC.getLine

split :: (a -> Bool) -> [a] -> [[a]]
split f [] = []
split f as = let
    (fo, la) = span f as
    las = case la of
        [] -> []
        xs -> split f $ tail xs
    in
        case fo of
            [] -> las
            xs -> xs : las

solve :: [Int] -> Bool
solve [] = True
solve heights = let
    newList = tail $ snd $ mapAccumL 
                                (\left right -> if left > right then (right, left - 1) else (right, left) ) 
                                0 heights
    leftAndRight = zip (0: newList) newList
    in
        all (\(left, right) -> right >= left) leftAndRight
    

main = do
    _ <- readInt
    heights <- readInts
    putStrLn $ if solve heights then "Yes" else "No"


