排列组合求助

P10986

#include<bits/stdc++.h>
#define int long long
#define mod 998244353
using namespace std;
int n = 0 , m = 0;
array<int , 100100> fact , fact_inv;
int quick_pow(int x , int y , int p)
{
    int res = 1;
    while(y)
    {
        if(y & 1)
        {
            res *= x;
            res %= p;
        }
        x *= x;
        x %= p;
        y /= 2;
    }
    return res;
}
int get_inv(int x)
{
    return quick_pow(x , mod - 2 , mod);
}
int C(int n , int m)
{
    return fact[n] * fact_inv[m] % mod * fact_inv[n - m] % mod;
}
signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    cin >> n >> m;
    fact[0] = 1;
    for(int i = 1;i <= n;++ i) fact[i] = fact[i - 1] * i % mod;
    for(int i = 0;i <= n;++ i) fact_inv[i] = get_inv(fact[i]);
    if(n - 4 * m <= 3)
    {
        cout << quick_pow(10 , n - 4 * m , mod) * C(n - 4 * m + m , m) % mod << endl;
        return 0;
    }
    else
    {
        cout << (quick_pow(10 , n - 4 * m , mod) * C(n - 4 * m + m , m) % mod - quick_pow(10 , n - 4 * m - 4 , mod) * C(n - 4 * m - 4 + m + 1 , m + 1) % mod + mod) % mod % mod << endl;
    }
    return 0;
}

这道题为什么不能用至少(m + 1)个的情况减去至少m个的情况?