#include<iostream>
#include<iomanip>
#include<cmath>
#include<string>
#include<algorithm>
#include<vector>
#include<math.h>
#include<stack>
using namespace std;
int main(void){
    int N; cin >> N;
    int X, Y, cnt = 0;
    for(int i = 0; i < N; i++){
        cin >> X >> Y;
        if(i == 0) {
            cnt = Y - X;
        }else{
            if(cnt == Y - X){
                continue;
            }else{
                cnt = -1;
                break;
            }
        }
    }
    if(cnt <= 0){
        cout << -1 << endl;
    }else{
        cout << cnt << endl;
    }
    return 0;
}