#include <iostream>
#include <atcoder/convolution>
#include <atcoder/modint>

using namespace std;
using namespace atcoder;
using mint = modint998244353;
int a[1010],b[1010];
mint fa[1010];
int main(){
    int i,j,h,w; cin >> h >> w;
    vector<mint> f = {1};
    fa[0] = 1;
    for(i=1;i<=w;i++) fa[i] = fa[i - 1]*i;
    for(i=0;i<h;i++){
        cin >> a[i] >> b[i];
        mint val1 = w + b[i],val2 = w + a[i] - 1;
        mint pr1 = 1,pr2 = 1;
        for(j=1;j<=w;j++){
            pr1 *= val1; pr1 /= j;
            pr2 *= val2; pr2 /= j;
            val1--; val2--;
        }
        vector<mint> g;
        g.push_back(pr1 - pr2);
        for(j=1;j<=w;j++){
            pr1 *= val1; pr1 /= (w + j);
            pr2 *= val2; pr2 /= (w + j);
            val1--; val2--;
            g.push_back((pr1 - pr2)/fa[j]);
        }
        // for(mint x:g) cout << x.val() << " ";
        // cout << endl;
        f = convolution(f,g);
        f.resize(w + 1);
    }
    cout << (fa[w]*f[w]).val() << endl;
}