#include <iostream>
using namespace std;
int main()
{
    int n; cin>>n;
    int x, y; cin>>x>>y;
    int res=x-y;
    while (--n) {
        cin>>x>>y;
        if (x-y!=res) res=-1;
    }
    cout<<(res>0 ? res : -1)<<endl;
}