#include <iostream>
#include <vector>
using namespace std;

int main(){
    int n;
    cin >> n;
    vector<int> x(n), y(n);
    for(int i = 0; i < n; i++) cin >> x[i] >> y[i];
    int ans = y[0]-x[0];
    for(int i = 0; i < n; i++){
        if(ans == -1) continue;
        else if(ans <= 0 || ans != y[i]-x[i]) ans = -1;
    }
    cout << ans << endl;
    return 0;
}