#include <bits/stdc++.h>
using namespace std;
void setup()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
}
struct DSU
{
int par[2000001], sz[2000001];
inline int Get(int inp)
{
return (par[inp] == inp ? inp : par[inp] = Get(par[inp]));
}
inline DSU()
{
for (int i = 0; i <= 2e6; ++i)
{
par[i] = i;
sz[i] = 1;
}
}
inline void Combine(int ina, int inb)
{
ina = Get(ina);
inb = Get(inb);
if (ina != inb)
{
if (sz[ina] < sz[inb])
{
swap(ina, inb);
}
sz[ina] += sz[inb];
par[inb] = ina;
}
}
} dsu;
struct SEGMENT_TREE
{
int tree[8000000];
inline void Update(int ind, int l, int r, int x, int v)
{
if (r < x || x < l)
{
return;
}
if (l == r)
{
tree[ind] = v;
return;
}
int m = (l + r) >> 1;
Update(ind << 1, l, m, x, v);
Update(ind << 1 | 1, m + 1, r, x, v);
tree[ind] = max(tree[ind << 1], tree[ind << 1 | 1]);
}
inline int Get(int ind, int l, int r, int x, int y)
{
if (r < x || y < l)
{
return 0;
}
if (x <= l && r <= y)
{
return tree[ind];
}
int m = (l + r) >> 1;
return max(Get(ind << 1, l, m, x, y), Get(ind << 1 | 1, m + 1, r, x, y));
}
} st;
int n, a[2000001], b, res = 1;
const int mod = 1e9 + 7;
set<int> s;
set<int>::iterator l, r;
vector<list<int>> all;
list<int> temp;
int main()
{
setup();
cin >> n;
for (int i = 0, j; i < n; ++i)
{
cin >> j >> a[j];
}
for (int i = 1; i <= 2 * n; ++i)
{
if (a[i] != 0)
{
s.insert(a[i]);
l = s.lower_bound(i);
r = s.lower_bound(a[i]);
b = st.Get(1, 1, 2 * n, i, a[i]);
if (i < b && b < a[i])
{
cout << 0;
return 0;
}
if (l != r)
{
r = prev(r);
st.Update(1, 1, 2 * n, a[i], (*r));
}
}
}
// list::splice() work very fast, read more on internet
for (int i = 1; i <= 2 * n; ++i)
{
if (a[i] != 0)
{
all.push_back({{a[i]}});
}
else
{
temp.clear();
while (true)
{
if (all.back().back() == i)
{
all.back().pop_back();
if (all.back().empty())
{
all.pop_back();
}
break;
}
dsu.Combine(i, all.back().back());
temp.splice(temp.begin(), all.back());
all.pop_back();
}
if (!temp.empty())
{
all.push_back({});
all.back().splice(all.back().begin(), temp);
}
}
}
for (int i = 1; i <= 2 * n; ++i)
{
if (a[i] != 0 && dsu.Get(a[i]) == a[i])
{
(res *= 2) %= mod;
}
}
cout << res;
return 0;
}
