#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

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

    int ans = 0;

    rep(i,n){
        int x, y;
        cin >> x >> y;
        if(i == 0){
            ans = y - x;
        }else{
            int tmp = y - x;
            if(tmp != ans){
                ans = -1;
            }
        }
    }

    if(ans <= 0)ans = -1;

    cout << ans << endl;
    return 0;
}