#include using namespace std; #define rep(i,n) for(int i=0;i<(n);++i) #define loop for(;;) #define trace(var) cerr<<">>> "<<#var<<" = "< inline ostream& operator<<(ostream&os, pair p) { return os << '(' << p.first << ", " << p.second << ')'; } template inline ostream& operator<<(ostream&os, tuple t) { return os << '(' << get<0>(t) << ", " << get<1>(t) << ", " << get<2>(t) << ')'; } template inline ostream& operator<<(ostream&os, vector v) { if (v.size() == 0) { return os << "(empty)"; } os << v[0]; for (int i=1, len=v.size(); i inline istream& operator>>(istream&is, vector&v) { rep (i, v.size()) is >> v[i]; return is; } // ^ > v < int dx[] = { -1, 0, 1, 0 }; int dy[] = { 0, 1, 0, -1 }; using vi = vector; using vvi = vector; using vd = vector; using vvd = vector; using vb = vector; int pt[200]; int sc[200]; int main() { cin.tie(0); ios::sync_with_stdio(0); cout.setf(ios::fixed); cout.precision(10); random_device dev; mt19937 rand(dev()); int n; cin >> n; int x, y; cin >> x >> y; int ans = y - x; rep (i, n-1) { int x, y; cin >> x >> y; if (ans != y-x) { cout << -1 << endl; return 0; } } if (ans < 0) { cout << -1 << endl; } else { cout << ans << endl; } return 0; }