description: You are given an array of n positive integers a_1, a_2, …, a_n. Your task is to calculate the number of arrays of n positive integers b_1, b_2, …, b_n such that: 
  
    * 1 ≤ b_i ≤ a_i for every i (1 ≤ i ≤ n), and 
    * b_i ≠ b_{i+1} for every i (1 ≤ i ≤ n - 1). 
  
  
  
  The number of such arrays can be very large, so print it modulo 998 244 353.
  
  Input
  
  The first line contains a single integer n (1 ≤ n ≤ 2 ⋅ 10^5) — the length of the array a.
  
  The second line contains n integers a_1, a_2, …, a_n (1 ≤ a_i ≤ 10^9).
  
  Output
  
  Print the answer modulo 998 244 353 in a single line.
  
  Examples
  
  Input
  
  
  3
  2 2 2
  
  
  Output
  
  
  2
  
  Input
  
  
  2
  2 3
  
  
  Output
  
  
  4
  
  Input
  
  
  3
  1 1 1
  
  
  Output
  
  
  0
  
  Note
  
  In the first test case possible arrays are [1, 2, 1] and [2, 1, 2].
  
  In the second test case possible arrays are [1, 2], [1, 3], [2, 1] and [2, 3].
solution: #include <bits/stdc++.h>
  using namespace std;
  using ll = long long;
  using uint = unsigned int;
  using pii = pair<int, int>;
  using pll = pair<ll, ll>;
  using ull = unsigned long long;
  using ld = long double;
  template <typename T>
  void _(const char* s, T h) {
    cerr << s << " = " << h << "\n";
  }
  template <typename T, typename... Ts>
  void _(const char* s, T h, Ts... t) {
    int b = 0;
    while (((b += *s == '(') -= *s == ')') != 0 || *s != ',') cerr << *s++;
    cerr << " = " << h << ",";
    _(s + 1, t...);
  }
  struct init {
    init() {
      cin.tie(0);
      iostream::sync_with_stdio(0);
      cout << fixed << setprecision(10);
      cerr << fixed << setprecision(5);
    }
    ~init() {}
  } init;
  template <typename T, typename U>
  void upx(T& x, U y) {
    if (x < y) x = y;
  }
  template <typename T, typename U>
  void upn(T& x, U y) {
    if (x > y) x = y;
  }
  mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
  const ll D = 4, dx[] = {+1, 0, -1, 0}, dy[] = {0, +1, 0, -1};
  const int32_t M = 998244353;
  int32_t rev(int32_t x, int32_t m) {
    if (x == 1) return 1;
    return (1 - (ll)rev(m % x, x) * m) / x + m;
  }
  struct mint {
    int32_t x;
    mint() : x(0) {}
    mint(int32_t xx) : x(xx % M) {
      if (x < 0) x += M;
    }
    mint(ll xx) : x(xx % M) {
      if (x < 0) x += M;
    }
    mint& operator+=(mint b) {
      if ((x += b.x) >= M) x -= M;
      return *this;
    }
    mint operator+(mint b) {
      mint res = *this;
      res += b;
      return res;
    }
    mint& operator-=(mint b) {
      if ((x -= b.x) < 0) x += M;
      return *this;
    }
    mint operator-(mint b) {
      mint res = *this;
      res -= b;
      return res;
    }
    mint& operator*=(mint b) {
      x = (ll)x * b.x % M;
      return *this;
    }
    mint operator*(mint b) {
      mint res = *this;
      res *= b;
      return res;
    }
    mint inverse() { return rev(x, M); }
    mint& operator/=(mint b) {
      x = (ll)x * rev(b.x, M) % M;
      return *this;
    }
    mint operator/(mint b) {
      mint res = *this;
      res /= b;
      return res;
    }
    mint operator^(ll w) {
      mint r = 1, a = *this;
      while (w) {
        if (w & 1) r *= a;
        a *= a;
        w /= 2;
      }
      return r;
    }
    mint operator^=(ll w) { return *this = (*this) ^ w; }
    mint operator^(mint w) { return *this ^ w.x; }
    mint operator^=(mint w) { return *this = (*this) ^ w; }
    mint operator-() const { return mint() - *this; }
    explicit operator bool() const { return x; }
  };
  istream& operator>>(istream& is, mint& p) {
    istream& res = is >> p.x;
    p.x %= M;
    return res;
  }
  ostream& operator<<(ostream& os, mint p) { return os << p.x; }
  template <typename T>
  mint operator+(T x, mint y) {
    return mint(x) + y;
  }
  template <typename T>
  mint operator-(T x, mint y) {
    return mint(x) - y;
  }
  template <typename T>
  mint operator*(T x, mint y) {
    return mint(x) * y;
  }
  template <typename T>
  mint operator/(T x, mint y) {
    return mint(x) / y;
  }
  template <typename T>
  mint operator^(T x, mint y) {
    return mint(x) ^ y;
  }
  mint abs(mint x) { return x; }
  const ll N = 505050;
  struct md {
    mint a, b;
    bool first;
  } mod[N * 4];
  mint t[N * 4], len[N * 4];
  ll sh = 1;
  void apply(ll v, md m) {
    if (m.first) {
      t[v] = m.b * len[v];
      mod[v] = m;
    } else {
      (t[v] *= m.a) += m.b * len[v];
      mod[v].a *= m.a;
      (mod[v].b *= m.a) += m.b;
    }
  }
  void push(ll v) {
    apply(v + v, mod[v]);
    apply(v + v + 1, mod[v]);
    mod[v] = {1, 0, 0};
  }
  void recalc(ll v) { t[v] = t[v + v] + t[v + v + 1]; }
  ll a[N];
  ll al[N];
  void build(ll n) {
    while (sh < n) sh *= 2;
    for (ll i = 0; i < (sh + sh); ++i) t[i] = 0;
    for (ll i = 0; i < (sh + sh); ++i) mod[i] = {1, 0, 0};
    for (ll i = 0; i < (n); ++i) {
      len[i + sh] = al[i];
      if (i) len[i + sh] -= al[i - 1];
    }
    for (ll i = sh - 1; i > 0; --i) len[i] = len[i + i] + len[i + i + 1];
  }
  ll l, r;
  md m;
  void upd(ll v, ll tl, ll tr) {
    if (l <= tl && tr <= r) {
      apply(v, m);
      return;
    }
    push(v);
    ll tm = (tl + tr) / 2;
    if (tm >= l) upd(v + v, tl, tm);
    if (tm + 1 <= r) upd(v + v + 1, tm + 1, tr);
    recalc(v);
  }
  void upd(ll _l, ll _r, md _m) {
    if (_l > _r) return;
    l = _l, r = _r, m = _m;
    upd(1, 0, sh - 1);
  }
  int32_t main() {
    ll n;
    cin >> n;
    for (ll i = 0; i < (n); ++i) {
      cin >> a[i];
      al[i] = a[i];
    }
    sort(al, al + n);
    ll m = unique(al, al + n) - al;
    build(m);
    ll ap = lower_bound(al, al + m, a[0]) - al;
    upd(0, ap, md{0, 1, 1});
    for (ll i = 1; i < n; ++i) {
      ll ai = lower_bound(al, al + m, a[i]) - al;
      mint tot = t[1];
      upd(0, ai, md{-1, tot, 0});
      upd(ai + 1, sh - 1, md{0, 0, 0});
      ap = ai;
    }
    cout << t[1] << '\n';
    return 0;
  }
  