#include<iostream>
#include<algorithm>
#include<vector>
using namespace std;
typedef long long ll;
#define chmin(a,b) a=min(a,b)
#define chmax(a,b) a=max(a,b)
#define rep(i,n) for(int i=0;i<n;i++)
#define mod 1000000007
#define mad(a,b) a=(a+b)%mod
#define N 510
#define X 1000010
vector<ll> dp[N];
ll a[N],b[N];
int main(){
ll n; cin>>n;
a[0]=b[0]=0;
dp[0].push_back(1);
for(int i=1;i<=n;i++){
cin>>a[i]>>b[i];
for(int j=a[i];j<=b[i];j++)dp[i].push_back(0);
for(int j=0;j<i;j++){
for(int k=a[j];k<=b[j];k++){
if(k<a[i])mad(dp[i][0],dp[j][k-a[j]]);
else if(k<b[i])mad(dp[i][k+1-a[i]],dp[j][k-a[j]]);
}
}
for(int j=a[i]+1;j<=b[i];j++)mad(dp[i][j-a[i]],dp[i][j-1-a[i]]);
}
ll ans=0;
for(int i=1;i<=n;i++){
for(int j=a[i];j<=b[i];j++){
//cout<<i<<" "<<j<<" "<<dp[i][j-a[i]]<<endl;
mad(ans,dp[i][j-a[i]]);
}
}
cout<<ans<<endl;
}
