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

// clear adj and visited vector declared globally after each test case
// check for long long overflow
// while adding and subs check if mod becomes -ve
// while using an integer directly in a builtin function add ll
// Mod wale question mein last mein if dalo ie. Ans<0 then ans+=mod;
// Dont keep array name as size or any other key word

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#define int long long
#define IOS                                                                    \\
  std::ios::sync_with_stdio(false);                                            \\
  cin.tie(NULL);                                                               \\
  cout.tie(NULL);                                                              \\
  cout.precision(dbl::max_digits10);
#define pb push_back
#define mod 998244353ll
#define lld long double
#define mii map<int, int>
#define mci map<char, int>
#define msi map<string, int>
#define pii pair<int, int>
#define ff first
#define ss second
#define all(x) (x).begin(), (x).end()
#define rep(i, x, y) for (int i = x; i < y; i++)
#define fill(a, b) memset(a, b, sizeof(a))
#define vi vector<int>
#define setbits(x) __builtin_popcountll(x)
#define print2d(dp, n, m)                                                      \\
  for (int i = 0; i <= n; i++) {                                               \\
    for (int j = 0; j <= m; j++)                                               \\
      cout << dp[i][j] << " ";                                                 \\
    cout << "\\n";                                                              \\
  }
typedef std::numeric_limits<double> dbl;
using namespace __gnu_pbds;
using namespace std;
typedef tree<int, null_type, less<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set;
const long long N = 200005, INF = 2000000000000000000;
lld pi = 3.1415926535897932;
int lcm(int a, int b) {
  int g = __gcd(a, b);
  return a / g * b;
}
int power(int a, int b, int p) {
  if (a == 0)
    return 0;
  int res = 1;
  a %= p;
  while (b > 0) {
    if (b & 1)
      res = (res * a) % p;
    b >>= 1;
    a = (a * a) % p;
  }
  return res;
}
int dp[N], ans[N];
int32_t main() {
  IOS;
  int n, k;
  cin >> n >> k;
  int l[k], r[k];
  for (int i = 0; i < k; i++)
    cin >> l[i] >> r[i];
  dp[0] = 1;
  ans[0] = 1;
  for (int i = 1; i < n; i++) {
    for (int j = 0; j < k; j++) {
      int m1 = i - r[j], m2 = i - l[j];
      if (m2 >= 0)
        ans[i] = (ans[i] + dp[m2]) % mod;
      m1--;
      if (m1 >= 0)
        ans[i] = (ans[i] - dp[m1] + mod) % mod;
    }
    dp[i] = (dp[i - 1] + ans[i]) % mod;
  }
  cout << ans[n - 1];
}

Error: WA

Fixed submission:

// clear adj and visited vector declared globally after each test case
// check for long long overflow
// while adding and subs check if mod becomes -ve
// while using an integer directly in a builtin function add ll
// Mod wale question mein last mein if dalo ie. Ans<0 then ans+=mod;
// Dont keep array name as size or any other key word

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#define int long long
#define IOS                                                                    \\
  std::ios::sync_with_stdio(false);                                            \\
  cin.tie(NULL);                                                               \\
  cout.tie(NULL);                                                              \\
  cout.precision(dbl::max_digits10);
#define pb push_back
#define mod 998244353ll
#define lld long double
#define mii map<int, int>
#define mci map<char, int>
#define msi map<string, int>
#define pii pair<int, int>
#define ff first
#define ss second
#define all(x) (x).begin(), (x).end()
#define rep(i, x, y) for (int i = x; i < y; i++)
#define fill(a, b) memset(a, b, sizeof(a))
#define vi vector<int>
#define setbits(x) __builtin_popcountll(x)
#define print2d(dp, n, m)                                                      \\
  for (int i = 0; i <= n; i++) {                                               \\
    for (int j = 0; j <= m; j++)                                               \\
      cout << dp[i][j] << " ";                                                 \\
    cout << "\\n";                                                              \\
  }
typedef std::numeric_limits<double> dbl;
using namespace __gnu_pbds;
using namespace std;
typedef tree<int, null_type, less<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set;
const long long N = 100005, INF = 2000000000000000000;
lld pi = 3.1415926535897932;
int lcm(int a, int b) {
  int g = __gcd(a, b);
  return a / g * b;
}
int power(int a, int b, int p) {
  if (a == 0)
    return 0;
  int res = 1;
  a %= p;
  while (b > 0) {
    if (b & 1)
      res = (res * a) % p;
    b >>= 1;
    a = (a * a) % p;
  }
  return res;
}
int dp[N], ans[N];
int32_t main() {
  IOS;
  int n, k;
  cin >> n >> k;
  int l[k], r[k];
  for (int i = 0; i < k; i++)
    cin >> l[i] >> r[i];
  dp[0] = 1;
  ans[0] = 1;
  for (int i = 1; i < n; i++) {
    for (int j = 0; j < k; j++) {
      int m1 = i - r[j], m2 = i - l[j];
      if (m2 >= 0)
        ans[i] = (ans[i] + dp[m2]) % mod;
      m1--;
      if (m1 >= 0)
        ans[i] = (ans[i] - dp[m1] + mod) % mod;
    }
    dp[i] = (dp[i - 1] + ans[i]) % mod;
  }
  cout << ans[n - 1];
}