#include<iostream>
#include<climits>
#include<cstdlib>
#include<algorithm>
#include<vector>
using namespace std;
int main(){
    int n;
    int ans;
    bool f=true;
    cin >> n;
    vector<int> x(n),y(n),d(n);
    for(int i=0;i<n;i++){
        cin >> x[i] >> y[i];
        d[i] = y[i]-x[i];
    }
    for(int i=0;i<n-1;i++){
        if(d[i]!=d[i+1]){
            f=false;
            break;
        }
    }
    if(f){
        if(d[0]<0)cout << -1 << endl;
        else cout << d[0] <<endl;
    }
    else cout << -1 << endl;
}