#include #define rep(i, n) for (int i = 0; i < int(n); i++) #define rrep(i, n) for (int i = int(n) - 1; i >= 0; i--) #define reps(i, n) for (int i = 1; i <= int(n); i++) #define rreps(i, n) for (int i = int(n); i >= 1; i--) #define repi(i, a, b) for (int i = int(a); i < int(b); i++) #define each(x, y) for (auto &x : y) #define all(a) (a).begin(), (a).end() #define bit(b) (1ll << (b)) #define uniq(v) (v).erase(unique(all(v)), (v).end()) #define rsort(v) sort(all(v)); reverse(all(v)) using namespace std; using i32 = int; using i64 = long long; using f80 = long double; using vi32 = vector; using vi64 = vector; using vf80 = vector; using vstr = vector; inline void yes() { cout << "Yes" << '\n'; exit(0); } inline void no() { cout << "No" << '\n'; exit(0); } inline i64 gcd(i64 a, i64 b) { if (min(a, b) == 0) return max(a, b); if (a % b == 0) return b; return gcd(b, a % b); } inline i64 lcm(i64 a, i64 b) { return a / gcd(a, b) * b; } void solve(); int main() { ios::sync_with_stdio(0); cin.tie(0); cout << fixed << setprecision(16); solve(); return 0; } template class pqasc : public priority_queue, greater> {}; template class pqdesc : public priority_queue, less> {}; template inline void amax(T &x, T y) { if (x < y) x = y; } template inline void amin(T &x, T y) { if (x > y) x = y; } template inline T power(T x, i64 n) { T r = 1; while (n > 0) { if (n & 1) r *= x; x *= x; n >>= 1; } return r; } template istream& operator>>(istream &is, vector &v) { for (auto &x : v) is >> x; return is; } template ostream& operator<<(ostream &os, const vector &v) { rep(i, v.size()) { if (i) os << ' '; os << v[i]; } return os; } void solve() { int N; cin >> N; int ans = 0; rep(i, N) { int x, y; cin >> x >> y; if (i == 0) ans = y - x; else if (ans != y - x) { cout << -1 << '\n'; return; } } if (ans > 0) cout << ans << '\n'; else cout << -1 << '\n'; }