#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i = (a); i < (b); ++i)
#define REP(i,n) FOR(i,0,n)
#define SZ(n) (int)(n).size()
#define ALL(n) (n).begin(), (n).end()
#define MOD 1000000007
using namespace std;
typedef long long LL;
typedef vector<int> VI;

int main() {
	int n;
	LL a[1010], b[1010];
	cin >> n;
	REP(i, n) cin >> a[i] >> b[i];
	LL sum[1010];
	int t = 0;
	REP(i, n) {
		sum[i] = a[i] + 4 * b[i];
		if (sum[i] > sum[t]) t = i;
	}
	LL ans = 0;
	REP(i, n) {
		if ((sum[t] - sum[i]) % 2 != 0) {
			ans = -1;
			break;
		}
		ans += (sum[t] - sum[i]) / 2;
	}
	cout << ans << endl;
	return 0;
}