#include<bits/stdc++.h>
#define int long long
using namespace std;
struct lan{
	int t,s;
}a[101010];
signed main(){
	int n,ans=0,mins=1000000000,cnt=0;
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>a[i].t>>a[i].s;
		if(ans>a[i].s){
			cout<<"-1";
			return 0;
		}
		ans+=a[i].t;
		if(a[i].s<mins){
			mins=a[i].s;
			cnt=i;
		}
	}
	cout<<mins-a[cnt].t;
	return 0;
}