#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using PAIR = pair<int, int>;
using PAIRLL = pair<ll,ll>;
using vi = vector<int>;
using vll = vector<ll>;
using vvi = vector<vi>;
using vs = vector<string>;
#define rep(i,n) for(int i=0;i<(n);i++)
#define INF 100000000
#define REP(i,n) for(ll i=0;i<ll(n);i++)
#define REPD(i,n) for(ll i=n-1;i>=0;i--)
#define FOR(i,a,b) for(ll i=a;i<=ll(b);i++)
#define FORD(i,a,b) for(ll i=a;i>=ll(b);i--)
#define FORA(i,I) for(const auto& i:I)
//x:コンテナ
#define ALL(x) x.begin(),x.end() 
#define SIZE(x) ll(x.size()) 
//定数
#define INF32 2147483647 //2.147483647×10^{9}:32bit整数のinf
#define INF64 9223372036854775807 //9.223372036854775807×10^{18}:64bit整数のinf
#define MOD 1000000007 //問題による
#define F first
#define S second
//出力(空白区切りで昇順に)
#define coutALL(x) for(auto i=x.begin();i!=--x.end();i++)cout<<*i<<" ";cout<<*--x.end()<<endl
#define coutALLn(x) for(auto i=x.begin();i!=--x.end();i++)cout<<*i<<endl;cout<<*--x.end()<<endl
ll myceil(ll a,ll b){return (a+(b-1))/b;}
ll myfloor(ll a,ll b){return a/b;}

int vector_finder(std::vector<int> vec, int number) {
  auto itr = std::find(vec.begin(), vec.end(), number);
  size_t index = std::distance( vec.begin(), itr );
  if (index != vec.size()) { // 発見できたとき
    return 1;
  }
  else { // 発見できなかったとき
    return 0;
  }
}

vector<ll> vector_find_ll(vector<ll> vec,ll num){
    int N = vec.size();
    vector<ll> match_num;
    rep(i,N){
        if (vec[i] == num){
            match_num.push_back(i);
        }
    }
    return match_num;
}

ll vector_max(vector<ll> vec){
    ll ret = -INF64;
    rep(i,vec.size()){
        ret = max(ret,vec[i]);
    }
    return ret;
}

ll vector_min(vector<ll> vec){
    ll ret = INF64;
    rep(i,vec.size()){
        ret = min(ret,vec[i]);
    }
    return ret;
}



int main(){
    int N ;
    cin >> N;
    int x,y,v0;
    cin >> x >> y;
    v0 = y-x;
    if (v0 <= 0){
        cout << -1 << endl;
        return 0;
    }
    bool f = true;
    rep(i,N-1){
        cin >> x >> y;
        if (v0 == y-x){
            continue;
        }
        else{
            f = false;
        }
    }
    if (f){
        cout << v0 << endl;
    }
    else{
        cout << -1 <<endl;
    }
}