#include<iostream>
#include<vector>
#include<algorithm>
#include<cstdio>
using namespace std;
int main(){
    long int N,x,y,num;
    cin >> N;
    cin >> x >> y;
    num = y-x;
    for(long int i=0;i<N-1;i++){
        cin >> x >> y;
        if((y-x)!=num||(y-x)<=0){
            cout << "-1";
            return 0;
        }
        num = y-x; 
    }
    cout << num;
    return 0;
}