#include <cstdio>
#include <cstring>
#include <cmath>
#include <climits>
#include <iostream>
#include <iomanip>
#include <list>
#include <stack>
#include <queue>
#include <vector>
#include <map>
#include <set>
#include <string>
#include <utility>
#include <algorithm>
#include <numeric>
#include <functional>

#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 (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
#define ALL(a) (a).begin(),(a).end()

using namespace std;
typedef long long ll;

int main(int argc, char *argv[])
{
  cin.tie(0);
  ios::sync_with_stdio(false);

  int N, d, x, y;
  cin >> N >> x >> y;
  d = y - x;
  if (d <= 0) {
    cout << -1 << endl;
    return 0;
  }
  REP(i,N-1) {
    cin >> x >> y;
    if (d != y - x) {
      cout << -1 << endl;
      return 0;
    }
  }
  cout << d << endl;
    
  return 0;
}