// 70 pts #include <iostream> #include <vector> #include <algorithm> #include <cstring> using namespace std; int nums; int arr[1005]; const int mod = 1'000'007; int memo[1005][1005][2]; int dp(int x, int mval, bool attop) { if (x == nums) return 1; if (memo[x][mval][attop] != -1) return memo[x][mval][attop]; int ans = 0; if (attop) { ans += dp(x + 1, max(arr[x], mval), 1); ans %= mod; ans += (arr[x] - 1) * dp(x + 1, mval, 0); ans %= mod; } else { ans += dp(x + 1, mval + 1, 0); ans %= mod; ans += mval * dp(x + 1, mval, 0); ans %= mod; } return memo[x][mval][attop] = ans; } int main() { ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0); cin >> nums; for (int i = 0; i < nums; i++) cin >> arr[i]; memset(memo, -1, sizeof(memo)); cout << dp(1, 1, 1); return 0; }