#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) begin(v),end(v)
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
using ll = long long;
using pii = pair<int, int>;
constexpr ll INF = 1ll<<30;
constexpr ll longINF = 1ll<<60;
constexpr ll MOD = 1000000007;
constexpr bool debug = false;
//---------------------------------//

int main() {
	int N;
	cin >> N;
	vector<int> len(N);
	bool ok = true;
	REP(i, N) {
		int a, b;
		scanf("%d %d", &a, &b);
		len[i] = a + b * 4;
		ok &= !(len[i] & 1) ^ (len[0] & 1);
	}
	ll ans = 0;
	int mx = *max_element(ALL(len));
	REP(i, N) ans += mx - len[i];
	if (!ok) puts("-1");
	else cout << ans / 2 << endl;
}