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#include <iostream>
#include <vector>
#include <array>

constexpr int maxn = 5e5 + 7, treeSize = 1 << 21, mod = 1e9 + 7;
int n;
std::pair<int, int> polki[maxn], gdzieSpada[maxn];
int ileNaMnieSpada[maxn];
int tree[treeSize];
long long presses;
std::array<long long, maxn> fact, oneOverFact;

void preprocessFact() {
//constexpr std::pair<std::array<long long, maxn>, std::array<long long, maxn>> preprocessFact() {
//    std::array<long long, maxn> fact = {0}, oneOverFact = {0};
    fact[0] = fact[1] = 1;
    oneOverFact[0] = oneOverFact[1] = 1;
    for(int i = 2; i <= n; i++) {
//    for(int i = 2; i <= maxn; i++) {
        fact[i] = fact[i - 1] * i % mod;

        long long toMult = fact[i];
        oneOverFact[i] = 1;
        int exp = mod - 2;
        while(exp) {
            if(exp & 1)
                oneOverFact[i] = (oneOverFact[i] * toMult) % mod;
            toMult = (toMult * toMult) % mod;
            exp >>= 1;
        }
    }
//    return {fact, oneOverFact};
}

long long newton(int a, int k) {
    return (fact[a] * oneOverFact[k] % mod) * oneOverFact[n - k] % mod;
}

void add(int l, int r, int x) {
    l += treeSize / 2; r += treeSize / 2;
    tree[l] = x;
    tree[r] = x;
    while(l / 2 != r / 2) {
        if(l % 2 == 0)
            tree[l + 1] = x;
        if(r % 2 == 1)
            tree[r - 1] = x;
        l /= 2; r /= 2;
    }
}
int max(int i) {
    i += treeSize / 2;
    int ans = tree[i];
    while(i) {
        ans = std::max(ans, tree[i]);
        i /= 2;
    }
    return ans;
}


void input() {
    scanf("%d", &n);
    int a, b;
    for(int i = 0; i < n; i++) {
        scanf("%d%d", &a, &b);
        polki[i] = {a, b};
    }
}

void calcGdzieSpada() {
    for(int i = 0; i < n; i++) {
        gdzieSpada[i].first = max(polki[i].first) - 1;
        gdzieSpada[i].second = max(polki[i].second) - 1;
        add(polki[i].first, polki[i].second, i + 1);
    }
}

bool vis[maxn];
void resetVis() {
    for(int i = 0; i < n; i++)
        vis[i] = false;
}
void dfsior(int v) {
    vis[v] = true;
    auto [l, r] = gdzieSpada[v];
    if(l >= 0 && !vis[l]) {
        ileNaMnieSpada[l]++;
        dfsior(l);
    }
    if(r >= 0 && !vis[r]) {
        ileNaMnieSpada[r]++;
        dfsior(r);
    }
}
void calcIleSpada() {
    for(int i = 0; i < n; i++) {
        resetVis();
        dfsior(i);
    }
}

void calcPresses() {
    for(int v = 0; v < n; v++) {
        auto &ile = ileNaMnieSpada[v];
        presses += (((newton(n, ile + 1) * fact[ile]) % mod) * fact[n - ile - 1]) % mod;
    }
}

long long answer() {
    return presses * oneOverFact[n] % mod;
}

int main() {
//    auto xd = preprocessFact(); fact = xd.first; oneOverFact = xd.second;
    input();
    preprocessFact();
    calcGdzieSpada();
    calcIleSpada();
    calcPresses();
    printf("%lld", answer());
    return 0;
}