#include <bits/stdc++.h>
using namespace std;
#define REP(i,a,b) for(int i=a;i<(int)b;i++)
#define rep(i,n) REP(i,0,n)
int main() {
  int n; cin >> n;
  int ans = 123456789;
  rep(i, n) {
    int a, b; cin >> a >> b;
    if(a>=b) { ans = -1; break; }
    if(ans==123456789) ans = b-a;
    else if(ans == b-a) {}
  else {ans=-1;break;}
  }
cout << ans << endl;

  return 0;
}