This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
//#pragma GCC optimize("Ofast")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 2e5 + 5;
const ll INF = 1e18 + 5;
const ll mod = 1e6 + 7;
const int tx[] = {1, -1, 0, 0}, ty[] = {0, 0, 1, -1};
const ll bs = 31;
#define int ll
int n, a[N], b[N], cnt, res[N];
int dp[2][1005][1005];
void rec(int cur = 1, int len = 1) {
    if (len == n) {
        cnt++;
        for (int i = 1; i <= n; i++) {
            if (a[i] != b[i])
                return;
        }
        cout << "CH ";
        cout << cnt % mod;
        exit(0);
    }
    for (int i = 1; i <= cur + 1; i++) {
        b[len + 1] = i;
        rec(max(cur, i), len + 1);
        b[len + 1] = 0;
    }
}
int calc(int id, int mx, int tp) {
//    cout << id << " " << mx << " " << tp << "\n";
    if (dp[tp][id][mx] != -1)
        return dp[tp][id][mx];
    if (id == 1)
        return dp[tp][id][mx] = calc(id + 1, mx + 1, 0);
    return dp[tp][id][mx] = (calc(id + 1, mx, 0) * mx % mod + calc(id + 1, mx + 1, 1)) % mod;
}
main() {
    cin >> n;
    memset(dp, -1, sizeof(dp));
    for (int i = 1; i <= n; i++) {
        cin >> a[i];
        dp[0][n][i] = i;
        dp[1][n][i] = 1;
    }
    b[1] = 1;
//    rec();
    int ans = 0;
    int mx = 0;
    for (int i = 1; i < n; i++) {
        for (int j = 1; j < a[i]; j++) {
//            cout << i + 1 << " ";
//            cout << calc(i + 1, max(mx + 1, j + 1), 0) << "\n";
            ans = (ans + calc(i + 1, max(mx + 1, j + 1), 0)) % mod;
        }
        mx = max(mx, a[i]);
    }
    cout << (ans + a[n]) % mod;
    return 0;
}
/**
4
1 2 2 3
*/
Compilation message (stderr)
teams.cpp:47:6: warning: ISO C++ forbids declaration of 'main' with no type [-Wreturn-type]
 main() {
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