Problem p02549 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
#define REP(i, n) for (int i = 0; i < n; i++)
#define RREP(i, n) for (int i = n; i >= 0; i--)
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define ALL(v) v.begin(), v.end()
#define RALL(v) v.rbegin(), v.rend()
using namespace std;
typedef long long ll;
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
// const ll INF = 1LL << 60;
// const int INF = 1001001001;

const int mod = 998244353;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return *this *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream &operator>>(istream &is, mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }
int main(void) {
  int n, k;
  cin >> n >> k;
  vector<int> l(k), r(k);
  REP(i, k) cin >> l[i] >> r[i];
  vector<mint> dp(n + 1);
  vector<mint> dpsum(n + 1);
  dp[1] = 1;
  dpsum[1] = 1;
  for (int i = 2; i <= n; i++) {
    REP(j, k) {
      //[l[j],r[j]]
      int li = i - r[j];
      int ri = i - l[j];
      if (ri < 0)
        continue;
      li = max(li, 1);
      dp[i] += dpsum[ri] - dpsum[li - 1]; // dp[li]~dp[ri]
    }
    dpsum[i] = dpsum[i - 1] + dp[i];
  }
  cout << dp[n] << endl;
  return 0;
}

Error: WA

Fixed submission:

#include <bits/stdc++.h>
#define REP(i, n) for (int i = 0; i < n; i++)
#define RREP(i, n) for (int i = n; i >= 0; i--)
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define ALL(v) v.begin(), v.end()
#define RALL(v) v.rbegin(), v.rend()
using namespace std;
typedef long long ll;
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
// const ll INF = 1LL << 60;
// const int INF = 1001001001;

const int mod = 998244353;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return *this *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream &operator>>(istream &is, mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

int main(void) {
  int n, k;
  cin >> n >> k;
  int l[k], r[k];
  REP(i, k) { cin >> l[i] >> r[i]; }
  set<int> s;
  REP(i, k) {
    for (int j = l[i]; j <= r[i]; j++)
      s.insert(j);
  }
  mint dp[n + 1];
  REP(i, n + 1) dp[i] = 0;
  dp[1] = 1;
  for (int i = 1; i <= n; i++) {
    for (auto j = s.begin(); j != s.end(); j++) {
      if (dp[i].x != 0)
        dp[i + *j] += dp[i];
    }
  }
  cout << dp[n];
  return 0;
}