#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
#define FOR(i,a,n) for(int i=a;i<(int)(n);i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(a) (a).begin(),(a).end()
#define MP(a,b) make_pair(a,b)
#define PB(a) push_back(a)
#define F first
#define S second
const int INF = 2000000000;
const int DX[4]={0,1,0,-1}, DY[4]={-1,0,1,0};
struct P{int x;int y;P(int X=0,int Y=0){x=X;y=Y;}};

int main() {
	int N;
	cin >> N;
	vector<int> a(N);
	vector<int> b(N);
	vector<int> top(N);
	int m=0;
	REP(i,N) {
		cin >> a[i] >> b[i];
		top[i] = a[i]+4*b[i];
		m = max(m, top[i]);
	}

	bool b1=false, b2=false;
	REP(i,N) {
		if(top[i] % 2 == 1) b1=true;
		else b2=true;
	}
	if(b1 && b2) {
		cout << -1 << endl;
		return 0;
	}

	int ans = 0;
	while(true) {
		bool b = false;
		REP(i,N) {
			if(m>top[i]) {
				b=true;
				while (true) {
					if(m<=top[i]) break;
					top[i]+=2;
					ans++;
				}
				m=top[i];
			}
		}
		if(!b) break;
	}
	cout << ans << endl;
	return 0;
}