#include <bits/stdc++.h>
#include <atcoder/convolution>
using namespace std;
const long long MOD = 998244353;
int main(){
  int H, W;
  cin >> H >> W;
  vector<int> A(H), B(H);
  for (int i = 0; i < H; i++){
    cin >> A[i] >> B[i];
  }
  vector<long long> inv(W * 2 + 1);
  inv[1] = 1;
  for (int i = 2; i <= W * 2; i++){
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
  }
  vector<long long> fact(W * 2 + 1);
  vector<long long> finv(W * 2 + 1);
  fact[0] = 1;
  finv[0] = 1;
  for (int i = 1; i <= W * 2; i++){
    fact[i] = fact[i - 1] * i % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
  vector<vector<long long>> f(H, vector<long long>(W + 1, 0));
  for (int i = 0; i < H; i++){
    long long a = 1, b = 1;
    for (int j = B[i] + 1; j <= B[i] + W; j++){
      b *= j;
      b %= MOD;
    }
    for (int j = A[i]; j <= A[i] + W - 1; j++){
      a *= j;
      a %= MOD;
    }
    for (int j = 0; j <= W && j <= B[i]; j++){
      int B2 = B[i] - j;
      f[i][j] = b * finv[W + j] % MOD;
      b *= B[i] - j;
      b %= MOD;
      if (A[i] > j){
        int A2 = A[i] - j - 1;
        f[i][j] += MOD - a * finv[W + j] % MOD;
        f[i][j] %= MOD;
        a *= A[i] - j - 1;
        a %= MOD;
      }
    }
  }
  for (int i = 0; i < H; i++){
    for (int j = 0; j <= W; j++){
      f[i][j] *= finv[j];
      f[i][j] %= MOD;
    }
  }
  vector<long long> g(W + 1, 0);
  g[0] = 1;
  for (int i = 0; i < H; i++){
    g = atcoder::convolution(g, f[i]);
    g.resize(W + 1);
  }
  cout << g[W] * fact[W] % MOD << endl;
}