#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll, ll> P;

#define EACH(i,a) for (auto& i : a)
#define FOR(i,a,b) for (ll i=(a);i<(b);i++)
#define RFOR(i,a,b) for (ll i=(b)-1;i>=(a);i--)
#define REP(i,n) for (ll i=0;i<(n);i++)
#define RREP(i,n) for (ll i=(n)-1;i>=0;i--)
#define debug(x) cout<<#x<<": "<<x<<endl
#define pb push_back
#define ALL(a) (a).begin(),(a).end()

const ll linf = 1e18;
const int inf = 1e9;
const double eps = 1e-12;
const double pi = acos(-1);

template<typename T>
istream& operator>>(istream& is, vector<T>& vec) {
	EACH(x,vec) is >> x;
	return is;
}
template<typename T>
ostream& operator<<(ostream& os, vector<T>& vec) {
	REP(i,vec.size()) {
		if (i) os << " ";
		os << vec[i];
	}
	return os;
}
template<typename T>
ostream& operator<<(ostream& os, vector< vector<T> >& vec) {
	REP(i,vec.size()) {
		if (i) os << endl;
		os << vec[i];
	}
	return os;
}
#define int ll
signed main() {
	std::ios::sync_with_stdio(false);
	std::cin.tie(0);
	int N; cin >> N;
	int r = 0;
	int ans = 0;
	REP(i, N) {
		int a, b; cin >> a >> b;
		int l = a + 4 * b;
		if (l > r) {
			if (i == 0 || (l-r)%2 == 0) {
				ans += (l-r)/2 * i;
				r = l;
			}
			else {
				cout << -1 << endl;
				return 0;
			}
		}
		else if ((r-l)%2 == 0) {
			ans += (r-l)/2;
		}
		else {
			cout << -1 << endl;
			return 0;
		}
	}
	cout << ans << endl;
}