#include <iostream>
using namespace std;

int main(){
   int n,ans;
   cin >> n;
   int x[n],y[n];
   for(int i = 0;i < n;i++) cin >> x[i] >> y[i];
   if(y[0] - x[0] <= 0) ans = -1;
   else ans = y[0] - x[0];
   for(int i = 1;i < n;i++) if(y[i] - x[i] <= 0 || ans != y[i] - x[i]) ans = -1;
   cout << ans << endl;
}