#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);
    for(int i=0;i<n;i++){
        cin >> x[i] >> y[i];
        ans = y[i]-x[i];
        if(ans<0){
            ans=-1;
            break;
        }
        if(i!=0&&ans!=y[i-1]-x[i-1]){
            ans = -1;
            break;
        }
    }
    cout << ans << endl;
}