#include <algorithm>
#include <vector>
#include <cfloat>
#include <string>
#include <cmath>
#include <set>
#include <cstdlib>
#include <map>
#include <ctime>
#include <iomanip>
#include <functional>
#include <deque>
#include <iostream>
#include <cstring>
#include <queue>
#include <cstdio>
#include <stack>
#include <climits>
#include <sys/time.h>
#include <cctype>

using namespace std;

typedef long long ll;

int main() {
  int n;
  cin >> n;
  int ans = 0;
  int x, y;
  cin >> x >> y;
  ans = y-x;
  if (ans <= 0) {
    std::cout << -1 << std::endl;
    return 0;
  }
  for (int i = 0; i < n-1; i++) {
    cin >> x >> y;
    if (ans != y-x) {
      std::cout << -1 << std::endl;
      return 0;
    }
  }
  std::cout << ans << std::endl;
  return 0;
}