n=int(input())
a=list(map(int,input().split()))
dp=[0]*n
mod=10**9+7
dp[0]=1
ary=[{} for i in range(n)]
base=0
for i in range(n):
  # マスiに到達するケースを加算
  if ary[i]:
    for k,v in ary[i].items():
      dp[i]=(dp[i]+v)%mod
      if i+k<n:
        if k in ary[i+k]:
          ary[i+k][k]+=v
        else:
          ary[i+k][k]=v
  dp[i]+=base
  dp[i]%=mod
  if i==n-1:break
  k=a[i]
  if k==1:
    base+=dp[i]
    base%=mod
  else:
    dp[i+1]+=dp[i]
    if i+k<n:
      if k in ary[i+k]:
        ary[i+k][k]+=dp[i]
        ary[i+k][k]%=mod
      else:
        ary[i+k][k]=dp[i]
print(dp[-1])