为什么一直TLE？？

#include <iostream>
#include <algorithm>

using namespace std;

typedef long long LL;

const int N = 5e6+10, mod = 998244353;


LL fact[N], infact[N];


int qmi(LL a, int k, int p)
{
    int res = 1;
    while (k)
    {
        if (k & 1) res = (LL)res * a % p;
        a = (LL)a * a % p;
        k >>= 1;
    }
    return res;
}


int main()
{
     int n,m;
    scanf("%d%d", &n,&m);
    fact[0] = infact[0] = 1;
    for (int i = 1; i <=m; i ++ )
    {
        fact[i] = (LL)fact[i - 1] * i % mod;
        infact[i] = (LL)infact[i - 1] * qmi(i, mod - 2, mod) % mod;
    }
    LL sum=0;
    while (n -- )
    {
        int a, b;
        scanf("%d%d", &a, &b);
      sum=sum^((LL)fact[a] * infact[b] % mod * infact[a - b] % mod);
    }
    cout<<sum<<endl;

    return 0;
}