#include <bits/stdc++.h>
#include <atcoder/all>

using namespace std;
using namespace atcoder;

struct Fast {
  Fast() {
    std::cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << setprecision(10);
  }
} fast;

#define rep(i, a, b) for (int(i) = (int)(a); (i) < (int)(b); (i)++)
#define rrep(i, a, b) for (int(i) = (int)(b)-1; (i) >= (int)(a); (i)--)

using ll = long long;
using mint = modint998244353;

int main() {
  int h, w;
  cin >> h >> w;

  int factsz = 2 * w + 1;
  vector<mint> fact(factsz), ifact(factsz);
  fact[0] = 1;
  rep(i, 1, factsz) fact[i] = fact[i - 1] * i;
  ifact[factsz - 1] = fact[factsz - 1].inv();
  rrep(i, 1, factsz) ifact[i - 1] = ifact[i] * i;

  vector<mint> f(w + 1, 0);
  f[0] = fact[w];
  rep(i, 0, h) {
    ll a, b;
    cin >> a >> b;
    vector<mint> g(w + 1);
    mint fallfact_a = 1, fallfact_b = 1;
    rep(i, 0, w) {
      fallfact_a *= w + a - 1 - i;
      fallfact_b *= w + b - i;
    }
    rep(i, 0, w + 1) {
      g[i] = (fallfact_b - fallfact_a) * ifact[i] * ifact[i + w];
      fallfact_a *= a - 1 - i;
      fallfact_b *= b - i;
    }
    f = convolution(f, g);
    f.resize(w + 1);
  }
  cout << f[w].val() << endl;
}