#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> iint;
typedef pair<ll,ll> llll;
#define ALL(x) (x).begin(),(x).end()
const ll zero = 0;
const ll one = 1;
const ll INF = 9223372036854775807; //10^18
const int inINF = 2147483647; //10^9
const ll MOD = 1000000007; //10^9+7
const ll MOD2 = 998244353;
void Yes() {printf("Yes\n");}
void No() {printf("No\n");}
void YES() {printf("YES\n");}
void NO() {printf("NO\n");}

int main(){
    int N;
    cin >> N;
    vector<ll> x(N), y(N);
    for (ll 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 != y[i] - x[i]){
            printf("%d\n", -1);
            return 0;
        }
    }
    printf("%d\n", ans);


}