MOD=10**9+7
ans=0
fact=[1]
for i in range(1,4*10**5+5):
fact.append(fact[-1]*i%MOD)
def c(m,n):
return fact[m]*pow(fact[m-n]*fact[n],-1,MOD)
n=int(input())
for i,a in enumerate(map(int,input().split())):
if a>0:
ans+=c(a+i,i+1)%MOD
ans%=MOD
print(ans)#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define sz(x) ((int) x.size())
#define all(x) x.begin(), x.end()
typedef long long int ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<pii> vii;
typedef vector<ll> vl;
const int M = 1e9 + 7;
const int maxn = 5e5 + 1;
ll fexp(ll a, ll b) {
if (b == 0) return 1;
ll f = fexp(a, b / 2);
f = (f * f) % M;
if (b % 2) f = (f * a) % M;
return f;
}
ll inv(ll n) {
return fexp(n, M - 2);
}
ll fat[maxn];
ll chs(ll n, ll k) {
return (fat[n] * inv((fat[n - k] * fat[k]) % M)) % M;
}
int main() {
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
fat[0] = 1;
for (int i = 1; i < maxn; ++i) {
fat[i] = (fat[i - 1] * i) % M;
}
int n; cin >> n; ++n;
vi a(n);
for (int &x : a) cin >> x;
ll ans = 0;
for (int i = 0; i < n; ++i) {
if (a[i] == 0) break;
ans = (ans + chs(i + a[i], a[i] - 1)) % M;
}
cout << ans << '\n';
return 0;
}
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