#include using namespace std; typedef long long ll; typedef unsigned long long ull; typedef pair iint; typedef pair llll; #define ALL(x) (x).begin(),(x).end() const ll zero = 0; const ll one = 1; const ll INF = 9223372036854775807; //10^18 const int inINF = 2147483647; //10^9 const ll MOD = 1000000007; //10^9+7 const ll MOD2 = 998244353; void Yes() {printf("Yes\n");} void No() {printf("No\n");} void YES() {printf("YES\n");} void NO() {printf("NO\n");} int main(){ int N; cin >> N; vector x(N), y(N); for (ll i = 0; i < N; i++) { cin >> x[i] >> y[i]; } int ans = y[0] - x[0]; for (int i = 0; i < N; i++) { if(ans != y[i] - x[i]){ printf("%d\n", -1); return 0; } } printf("%d\n", ans); }