#include <iostream>
#include <vector>

using namespace std;

int main(){
    int N;
    cin >> N;
    int a, b;
    cin >> a >> b;
    int ans = b - a;
    if(ans <= 0) ans = -1;
    for(int i = 0; i < N-1; i++){
        cin >> a >> b;
        if(b - a != ans || b - a <= 0){
            ans = -1;
            break;
        }
    }
    cout << ans << endl;
    return 0;
}