#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <algorithm>
#include <sstream>
#include <map>
#include <set>
using namespace std;


#define REP(i,a,b) for(i=a;i<b;i++)
#define rep(i,n) REP(i,0,n)

typedef long long ll;
typedef unsigned long long ull;
typedef long double lb;
/* ここからが本編 */


int main(void)
{
   int i,j,k,l;
   int n;
   int ans;
   int x,y;
   cin >> n;
   cin >> x >> y;
   ans = y - x;
   if(ans < 1) ans = -1;
   for(i=1;i<n;i++) {
      cin >> x >> y;
      if(ans != (y - x)) {
         ans = -1;
      }
   }

   cout << ans << endl;
   return 0;
}