#include using namespace std; const int MOD = (1e9+7); void printmat(const vector>& mat) { for (auto row : mat) { for (auto elem : row) cout << elem << " "; cout << endl; } } void printv(const vector& v) { for (auto elem : v) cout << elem << " "; cout << endl; } void printd(const deque& v) { for (auto elem : v) cout << elem << " "; cout << endl; } void printvp(const vector>& vp) { for (auto pr : vp) { cout << pr.first << ", " << pr.second; cout << endl; } } void printvs(const vector>& vs) { for (auto row : vs) { for (auto elem : row) cout << elem << ", "; cout << endl; } } void printht(const unordered_map& ht) { for (auto elem : ht) cout << elem.first << " : " << elem.second << endl; } void printmp(const map& ht) { for (auto elem : ht) cout << elem.first << " : " << elem.second << endl; } void printst(const set& st) { for (auto elem : st) cout << elem << " "; cout << endl; } bool isPrime(long long n) { if (n <= 1) return false; if (n <= 3) return true; if (n % 2 == 0 || n % 3 == 0) return false; for (long long i = 5; i * i <= n; i = i + 6) if (n % i == 0 || n % (i + 2) == 0) return false; return true; } map primeFactors(long long n) { map ans; while (n % 2 == 0) { ans[2]++; n = n/2; } for (long long i = 3; i*i <= (n); i = i + 2) { while (n % i == 0) { ans[i]++; n = n/i; } } if (n > 2) ans[n]++; return ans; } int find_f(const vector& uf, int i) { while (uf[i]!=i) i = uf[i]; return i; } bool union_f(vector& uf, vector& sz, int a, int b) { a = find_f(uf, a); b = find_f(uf, b); //cout << "a, b = " << a << ", " << b << endl; if (a==b) return false; if (sz[a] < sz[b]) { //cout << "sz[a], sz[b] = " << sz[a] << ", " << sz[b] << endl; //cout << "a, b = " << a << ", " << b << endl; swap(a,b); //cout << "a, b = " << a << ", " << b << endl; } sz[a] += sz[b]; uf[b] = a; return true; } int main() { ios_base::sync_with_stdio(false); cin.tie(NULL); int T=1, caseIdx=0; //cin >> T; while (T--) { //caseIdx++; int n, ans=-1; cin >> n; set st; for (int i=0; i> x >> y; st.insert(y-x); } if (st.size()==1 && *st.begin()>0) ans = *st.begin(); cout << ans << endl; //cout << "Case #" << caseIdx << ": " << s << endl; } }