{-# LANGUAGE BangPatterns, MultiWayIf #-}
import Control.Arrow
import Control.Monad.Fix
import Data.Int
import Data.Foldable
import qualified Data.Vector as V
import qualified Data.Vector.Unboxed as VU
import qualified Data.ByteString.Char8 as B
import Debug.Trace

solve :: Int -> Int -> VU.Vector Int32 -> Int32
solve !n !k !an = VU.last dp
 where
  dp :: VU.Vector Int32
  dp = VU.foldl'
    (\(!dpi) (!i) -> VU.scanl'
      (\(!dpij) (!j) ->
        let !p = if j - fromIntegral (an VU.! i) - 1 >= 0
              then dpi VU.! (j - fromIntegral (an VU.! i) - 1)
              else 0
            !q = dpi VU.! j
            !a = dpij + q - p
        in  a `mod` (10 ^ 9 + 7)
      )
      1
      (VU.generate k (+ 1))
    )
    (VU.generate (k + 1) (\i -> if i == 0 then 1 else 0))
    (VU.generate n id)

main = do
  let readInt :: Integral a => B.ByteString -> Maybe (a, B.ByteString)
      readInt = fmap (first fromIntegral . second B.tail) . B.readInt

  (n, k) <-
    (\(!vec) -> (vec VU.! 0, vec VU.! 1)) . VU.unfoldrN 2 readInt <$> B.getLine
  an <- VU.unfoldrN n readInt <$> B.getLine
  print $ solve n k an
