#include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include template inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; } template inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; } //constexpr long long MAX = 5100000; constexpr long long INF = 1LL << 60; constexpr int inf = 1000000007; constexpr long long mod = 1000000007LL; //constexpr long long mod = 998244353LL; const long double PI = acos((long double)(-1)); using namespace std; typedef unsigned long long ull; typedef long long ll; typedef long double ld; int main() { /* cin.tie(nullptr); ios::sync_with_stdio(false); */ int n; cin >> n; vector x(n), y(n); for (int i = 0; i < n; i++) scanf("%d %d", &x[i], &y[i]); int res = y[0] - x[0]; for (int i = 1; i < n; i++) { if (res != y[i] - x[i]) { puts("-1"); return 0; } } cout << res << endl; return 0; }