#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define chmin(x,y) x = min((x),(y));
#define chmax(x,y) x = max((x),(y));
using namespace std;
using ll = long long ;
using P = pair<int,int> ;
using pll = pair<long long,long long>;
const int INF = 1e9;
const long long LINF = 1e17;
const int MOD = 1000000007;
//const int MOD = 998244353;
const double PI = 3.14159265358979323846;

int main(){
    int n;
    cin >> n;
    int ans = -1;
    rep(i,n){
        int a,b;
        cin >> a >> b;
        if(ans==-2) continue;
        if(ans==-1){
            ans = b-a;
            if(ans<=0) ans = -2;
        }
        else {
            if(ans != b-a){
                ans = -2;
            }
        }
    }
    if(ans==-2) cout << -1 << endl;
    else cout << ans << endl;
    return 0;
}