#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); i++)
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<ll> vll;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<vector<ll>> vvll;
const ll inf = 1e16;
const ll md = 1000000007;

int main() {
  int n,x,y;
  cin>>n>>x>>y;
  int ans=y-x;
  if(ans<=0){
    puts("-1");
    return 0;
  }
  rep(i,n-1){
    cin>>x>>y;
    if(y-x!=ans){
      puts("-1");
      return 0;
    }
  }
  cout<<ans<<endl;
  return 0;
}