#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <queue>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;

static const double EPS = 1e-8;
static const double PI = 4.0 * atan(1.0);
static const double PI2 = 8.0 * atan(1.0);

#define REP(i,n)	for(int i=0;i<(int)n;++i)
#define ALL(c)		(c).begin(),(c).end()
#define CLEAR(v)	memset(v,0,sizeof(v))
#define MP(a,b)		make_pair((a),(b))
#define ABS(a)		((a)>0?(a):-(a))
#define FOR(i,s,n)	for(int i=s;i<(int)n;++i)

int N, a, b;
int w[1000];

int main(int argc, char **argv) {
	cin >> N;
	int m = 0;
	bool odd = false, even = false;
	REP(i, N) {
		cin >> a >> b;
		w[i] = a + 4 * b;
		m = max(m, w[i]);
		if (w[i] % 2 == 0) even = true;
		else odd = true;
	}
	if (even && odd) {
		cout << -1 << endl;
	}
	else {
		ll res = 0;
		REP(i, N) res += (m - w[i]) / 2;
		cout << res << endl;
	}
	return 0;
}