#include <iostream>
using namespace std;
int main(void){
    int n;
    cin >> n;
    int x,y;
    cin >> x >> y;
    int sm = y - x;
    
    for(int i = 1; i < n; i++) {
        cin >> x >> y;
        int ans = y - x;
        if(sm != ans){
            cout << -1;
            return 0;
        }
    }
    if(sm < 0)cout << -1;
    else cout << sm;
    
    
}