#include<cstdio>

#include<iostream>

#include<algorithm>

#include<cmath>

#include<cstring>

#include<string>

#include<queue>

#include<vector>

#define inf 0x3f3f3f3f

#define debug(x) cout<<"-----"<<" x = "<<x<<"-----"<<endl

using namespace std;

typedef long long ll;

const int maxn = 5e3 + ;

const ll mod = 1e9 + ;

ll dp[maxn];

int p[maxn];

int main()

{

    int n;

    scanf("%d", &n);

    for (int i = ; i <= n; i++) scanf("%d", &p[i]);

    dp[] = ;

    dp[] = ;

    for(int i=;i<=n;i++)

    {

        dp[i+] = ( * dp[i]%mod - dp[p[i]]%mod + +mod)%mod;

    }

    printf("%lld\n", dp[n + ]%mod);

    return ;

}