#include <bits/stdc++.h> 

using i64 = long long; 
using u64 = unsigned long long; 
using u32 = unsigned; 

using u128 = unsigned __int128; 
using i128 = __int128; 

void solve() {
	int N; std::cin >> N;

	int X, Y; std::cin >> X >> Y;
	int ans = Y - X;
	if(ans <= 0) {
		std::cout << -1;
		return;
	}
	for(int i = 1; i < N; i ++) {
		std::cin >> X >> Y;
		int Z = Y - X;
		if(Z <= 0 || Z != ans) {
			std::cout << -1;
			return;
		}
	}

	std::cout << ans;
} 

int main() { 
	std::ios::sync_with_stdio(false); 
	std::cin.tie(nullptr); 

	int T = 1; 
	//std::cin >> T; 

	while (T--) { 
		solve(); 
	} 
	return 0; 
}