#define _CRT_SECURE_NO_WARNINGS #include <stdio.h> #include <algorithm> #include <utility> #include <functional> #include <cstring> #include <queue> #include <stack> #include <math.h> #include <iterator> #include <vector> #include <string> #include <set> #include <math.h> #include <iostream> #include<map> #include <iomanip> #include <stdlib.h> #include <list> #include <typeinfo> #include <list> #include <set> using namespace std; #define MAX_MOD 1000000007 #define REP(i,n) for(long long i = 0;i < n;++i) #define LONG_INF 100000000000000 int main() { int n; cin >> n; vector<long long> s; REP(i, n) { long long a, b; cin >> a >> b; a += b * 4; s.push_back(a); } sort(s.begin(), s.end(), greater<long long>()); long long ans = 0; for (int i = 1;i < n;++i) { if ((s[0]-s[i]) % 2 == 1) { cout << -1 << endl; return 0; } ans += (s[0]-s[i]) / 2; } cout << ans << endl; return 0; }