#include <map>
#include <set>
#include <list>
#include <cmath>
#include <queue>
#include <stack>
#include <cstdio>
#include <string>
#include <vector>
#include <complex>
#include <cstdlib>
#include <cstring>
#include <numeric>
#include <sstream>
#include <iostream>
#include <algorithm>
#include <functional>
  
#define mp       make_pair
#define pb       push_back
#define all(x)   (x).begin(),(x).end()
#define rep(i,n) for(int i=0;i<(n);i++)
  
using namespace std;
  
typedef    long long          ll;
typedef    unsigned long long ull;
typedef    vector<bool>       vb;
typedef    vector<int>        vi;
typedef    vector<vb>         vvb;
typedef    vector<vi>         vvi;
typedef    pair<int,int>      pii;
  
const int INF=1<<29;
const double EPS=1e-9;
  
const int dx[]={1,0,-1,0,1,1,-1,-1},dy[]={0,-1,0,1,1,-1,-1,1};


int main() {
    int N;
    cin >> N;
    std::vector<int> x;
    std::vector<int> y;
    for (int i = 0; i < N; i++) {
        int temp1, temp2;
        cin >> temp1 >> temp2;
        x.push_back(temp1);
        y.push_back(temp2);
    }
    int prek = 0;
    for (int i = 0; i < N; i++) {
        int k = y[i] - x[i];
        if (i != 0 && prek != k) {
            cout << "-1" << endl;
            return 0;
        }
        if (k <= 0) {
            cout << "-1" << endl;
            return 0;
        }
        prek = k;
    }
    cout << prek << endl;
    return 0;
}