#include <bits/stdc++.h>
using namespace std;
const int md = int(1e9) + 7;
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n;
cin >> n;
vector<int> a(n + 1), b(n + 1);
for (int i = 1; i <= n; i++) {
cin >> a[i] >> b[i];
}
vector<vector<int>> dp(n + 1);
for (int i = 0; i <= n; i++) {
dp[i].resize(b[i] - a[i] + 1);
}
dp[0][0] = 1;
for (int i = 1; i <= n; i++) {
for (int j = 0; j < i; j++) {
for (int x = a[j]; x <= b[j]; x++) {
if (x + 1 <= b[i]) {
int k = max(0, x + 1 - a[i]);
dp[i][k] += dp[j][x - a[j]];
dp[i][k] %= md;
}
}
}
for (int x = a[i] + 1; x <= b[i]; x++) {
dp[i][x - a[i]] += dp[i][x - 1 - a[i]];
dp[i][x - a[i]] %= md;
}
}
int ans = 0;
for (int i = 1; i <= n; i++) {
for (int x = a[i]; x <= b[i]; x++) {
ans += dp[i][x - a[i]];
ans %= md;
}
}
cout << ans << '\n';
return 0;
}
