#include <bits/stdc++.h>

using ll = std::int64_t;

std::tuple<ll, ll, ll> extgcd(ll a, ll b){
    ll s1 = 1, t1 = 0, s2 = 0, t2 = 1;
    while(b != 0){
        std::tie(s1, t1, s2, t2) = std::make_tuple(s2, t2, s1 - (a / b) * s2, t1 - (a / b) * t2);
        std::tie(a, b) = std::make_tuple(b, a % b);
    }
    return std::make_tuple(s1, t1, a);
}

ll inv(ll a, ll n){
    ll inv_a;
    std::tie(inv_a, std::ignore, std::ignore) = extgcd(a, n);
    inv_a %= n;
    if(inv_a < 0){inv_a += n;}
    return inv_a;
}

int main(){
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    int N;
    std::cin >> N;

    std::vector<int> l(N), r(N);
    for(int i=0;i<N;i++){
        std::cin >> l[i] >> r[i];
    }

    constexpr ll MOD = 998'244'353;
    ll res = 1;
    ll inv_n1 = inv(r[0] - l[0] + 1, MOD);
    for(int i=1;i<N;i++){
        ll inv_ni = inv(r[i] - l[i] + 1, MOD);

        int l1 = std::max(l[i], r[0] + 1);
        if(l1 <= r[i]){
            res += 1ll * (r[i] - l1 + 1) * inv_ni;
            if(res >= MOD){res -= MOD;}
        }

        int l2 = std::max(l[i], l[0]);
        int r2 = std::min(r[i], r[0]);
        if(l2 <= r2){
            res += (1ll * (r2 - l[0] + l2 - l[0]) * (r2 - l2 + 1) / 2) % MOD * inv_n1 % MOD * inv_ni % MOD;
            if(res >= MOD){res -= MOD;}
        }
    }

    std::cout << res << std::endl;
}