#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define REP(i, n)           for(int(i)=0;(i)<(n);++(i))
#define FOR(i, f, t)        for(int(i)=(f);(i)<(t);(++i))

int main(){
    do { cin.tie(0); ios_base::sync_with_stdio(false); } while(0);
    int N;
    cin >> N;
    ll x,y;
    cin >> x >> y;
    ll z = y - x;
    ll res = z;
    if(z < 1) res = -1;
    if(z > 0){
        REP(i,N-1){
            cin >> x >> y;
            if(z + x != y) res = -1;
        }
    }
    cout << res << endl;
}