#include using namespace std; #ifdef DEBUG_ #include #else #define dump(...) #endif #define FastIO cin.tie(nullptr), ios_base::sync_with_stdio(false); #define rep(i, n) for (int i = 0; i < (int)(n); ++i) #define output(msg) cout << (msg) << '\n' #define die(msg) \ do { \ cout << msg << endl; \ exit(0); \ } while (0) #define all(k) k.begin(), k.end() #define INFi 1 << 30 #define INFll 1LL << 60 template bool chmax(T& a, const T& b) { return ((a < b) ? (a = b, true) : (false)); } template bool chmin(T& a, const T& b) { return ((a > b) ? (a = b, true) : false); } using llint = long long int; int main() { FastIO; int n; cin >> n; set st; rep(_, n) { int x, y; cin >> x >> y; st.insert(y - x); } if (st.size() == 1) { int num; for (auto key : st) num = key; if (num >= 0) output(num); else output(-1); } else { output(-1); } }