#include <iostream>
using namespace std;

int main()
{
    int n;
    cin >> n;

    int x[1000],y[1000];
    int ans,flag = 0;
    for(int i = 0; i < n; i++){
        cin >> x[i] >> y[i];
        if(i > 0 && (y[i]-x[i] != y[i-1]-x[i-1]))
            flag = 1;
    }

    if(flag == 0 && y[0]-x[0] > 0)
        cout << y[0]-x[0] << endl;
    else
        cout << -1 << endl;

    return 0;
}