#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h> 
 
using namespace std;

typedef unsigned long long ull; 
typedef long long ll;
typedef long double ld;
typedef pair<int,int> p32;
typedef pair<ll,ll> p64;
typedef pair<double,double> pdd;
typedef vector<ll> v64;
typedef vector<int> v32;
typedef vector<vector<int> > vv32;
typedef vector<vector<ll> > vv64;
typedef vector<vector<p64> > vvp64;
typedef vector<p64> vp64;
typedef vector<p32> vp32;
typedef vector<bool> vb;
typedef vector<vector<bool> > vbb;

//ll MOD = 1e9+7;
double eps = 1e-12;
#define forn(i,e) for(int i = 0; i < e; i++)
#define forsn(i,s,e) for(ll i = s; i < e; i++)
#define rforn(i,s) for(ll i = s; i >= 0; i--)
#define rforsn(i,s,e) for(ll i = s; i >= e; i--)
#define ln "\n"
#define dbg(x) cout<<#x<<" = "<<x<<ln
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define INF 2e18
#define fast_cin() ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((ll)(x).size())
#define pob pop_back


//#include <conio.h>
void setIO(string s) { // the argument is the filename without the extension
freopen((s+".in").c_str(),"r",stdin);
freopen((s+".out").c_str(),"w",stdout);
}

int main(){
    int tc;
    cin>>tc;
    bool flag = true;
    set<int> a;
    while(tc--){
        int x,y;
        cin>>x>>y;
        int diff = y-x;
        if(diff>0){
            if(sz(a)==0)    a.insert(diff);
            else{
                if(a.count(diff)==0){
                    flag = false;
                }
                a.insert(diff);
            }
        }
        else flag = false;
    }
    
    if(!flag)   cout<<-1<<ln;
    else{
        v32 b(all(a));
        cout<<b[0]<<ln;
    }
    return 0;
}