#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int md = (int)1e9 + 7;
int main() {
ios::sync_with_stdio(false); cin.tie(nullptr);
int n;
cin >> n;
vector<int> a(n), b(n);
for (int i = 0; i < n; i++)
cin >> a[i] >> b[i];
vector<vector<int>> dp(n);
for (int i = 0; i < n; i++)
dp[i].assign(b[i] - a[i] + 2, !i);
for (int i = 1; i < n; i++) {
for (auto j : dp[i - 1])
dp[i][0] = (dp[i][0] + j) % md;
for (int j = a[i]; j <= b[i]; j++) {
dp[i][j - a[i] + 1] = dp[i - 1][0];
for (int k = a[i - 1]; k <= min(j - 1, b[i - 1]); k++)
dp[i][j - a[i] + 1] = (dp[i][j - a[i] + 1] + dp[i - 1][k - a[i - 1] + 1]) % md;
}
}
int ans = -1;
for (auto i : dp[n - 1])
ans = (ans + i) % md;
cout << ans;
}
