#include <bits/stdc++.h>
using namespace std;

#define BIT(x) (1ull << (x))
#define FOR(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define REP(i, n) FOR(i, 0, n)
#define RFOR(i, a, b) for (int i = (int)(b) - 1; i >= (int)(a); i--)
#define RREP(i, n) RFOR(i, 0, n)
#define EACH(i, v) for (__typeof((v).begin()) i = (v).begin(); i != (v).end(); ++i)
#define ALL(v) (v).begin(), (v).end()
#define RALL(v) (v).rbegin(), (v).rend()
#define UNIQ(v) do {sort(ALL(v)); (v).erase(unique(ALL(v)), (v).end());} while (0)
#define PB push_back
#define MP make_pair
#define SIZE(v) ((int)(v).size())
#define DEBUG(x) cout << #x << " = " << x << endl



int main()
{
	int n;
	int c[1000];
	
	scanf("%d", &n);
	int cnt = 0;
	int maxi = 0;
	REP(i, n){
		int a, b;
		scanf("%d %d", &a, &b);
		c[i] = a + b * 4;
		maxi = max(maxi, c[i]);
		cnt += (c[i] & 1);
	}
	if (cnt != 0 && cnt != n){
		puts("-1");
		return 0;
	}
	long long res = 0;
	REP(i, n){
		res += (maxi - c[i]) / 2;
	}
	printf("%lld\n", res);
}