#include <iostream>
#include <stdio.h>
using namespace std;

int main(int argc, char const* argv[])
{
    int n;
    cin>>n;
    int x,y,z;
    cin>>x>>y;
    z=y-x;
    for (int i = 0; i < n-1; i++) {
        int a,b,c;
        cin>>a>>b;
        c=b-a;
        if(c!=z) {
            cout<<"-1"<<endl;
            return 0;
        }
    }
    if(z<=0) {
        cout<<"-1"<<endl;
        return 0;
    }
    cout<<z<<endl;
    return 0;
}