求问取模值为何可能是负数？

#include<iostream>
#include<queue>
#include<algorithm>
#include<cstring>
#include<vector>
#include<set>
using namespace std;
#define int long long

const int N = 1.6e14 + 10, mod = 998244353;

int le, ri;

int calc(int p){
    int res = 0;
    for(int l=1, r;l<=p;l=r+1){
        r = p / (p / l);
        res = (res + ((r - l + 1) % mod) * ((p / l) % mod) % mod) % mod;
    }
    return res;
}

signed main(){
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);
    cin >> le >> ri;
    cout << (calc(ri) - calc(le - 1) + mod) % mod;
    return 0;
}