#include<bits/stdc++.h>

 #define pa pair<int,int>

 #define CLR(a,x) memset(a,x,sizeof(a))

 using namespace std;

 typedef long long ll;

 const int maxn=1e3+,P=;

 inline ll rd(){

     ll x=;char c=getchar();int neg=;

     while(c<''||c>''){if(c=='-') neg=-;c=getchar();}

     while(c>=''&&c<='') x=x*+c-'',c=getchar();

     return x*neg;

 }

 ll f[maxn],c[maxn][maxn];

 int a[maxn],N;

 int main(){

     int i,j,k;

     N=rd();

     for(i=;i<=N;i++) a[i]=rd();

     c[][]=c[][]=;

     for(i=;i<=N;i++){

         for(j=;j<=i;j++){

             if(j) c[i][j]=(c[i-][j-]+c[i-][j])%P;

             else c[i][j]=;

         }

     }

     for(i=N;i;i--){

         if(a[i]<=||i+a[i]>N) continue;

         int s=;

         for(j=N;j>=i+a[i]+;j--){

             f[i]=(f[i]+c[j-i-][a[i]]*f[j])%P;

         }

         f[i]=(f[i]+c[N-i][a[i]])%P;

     }

     ll ans=;

     for(i=;i<=N;i++) ans+=f[i],ans%=P;

     printf("%d\n",ans);

     return ;

 }