#include <bits/stdc++.h>
using namespace std;

//--------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define RFOR(i,a,b) for(int i=(b)-1;i>=(a);i--)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) RFOR(i,0,n)
#define LL long long
#define INF INT_MAX/3

//--------------------------------------------
const double EPS = 1e-14;
const double PI  = acos(-1.0);

//--------------------------------------------

int main(){
  int n;
  int x,y;

  scanf("%d",&n);

  int ans;
  REP(i, n) {
    scanf("%d %d",&x, &y);
    if (i == 0) {
      if (x > y) ans = -1;
      else ans = y - x;
    } else if (y - x != ans) ans = -1;
  }

  printf("%d\n",ans);
}