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

#define FOR(i, a, b) for(int i = (a); i < (b); i++)
#define RFOR(i, a, b) for(int i = (a) - 1; i >= (b); i--)
#define SZ(a) int(a.size())
#define ALL(a) a.begin(), a.end()
#define PB push_back
#define MP make_pair
#define F first
#define S second

typedef long long LL;
typedef vector<int> VI;
typedef pair<int, int> PII;
typedef double db;

const int C = 20;

int main()
{
	ios::sync_with_stdio(0); 
	cin.tie(0);
	int n;
	cin >> n;
	VI a(n), c(n), l(C, n), r(C, 0), cnt(C, 0);
	vector<VI> vec(n);
	FOR(i, 0, n)
	{
		cin >> a[i] >> c[i];
		a[i]--;
		c[i]--;
		l[c[i]] = min(l[c[i]], i);
		r[c[i]] = i;
		cnt[c[i]]++;
	}
	FOR(i, 0, C)
	{
		if (cnt[i] > 0 && cnt[i] != r[i] - l[i] + 1)
		{
			cout << "0\n";
			return 0;
		}
	}
	FOR(i, 0, n)
	{
		for (int x = 1; ; x = a[i] / (a[i] / x) + 1)
		{
			vec[i].PB(x);
			if (x > a[i])
				break;
		}
	}
	vector<PII> events;
	FOR(i, 0, n - 1)
	{
		VI cur;
		cur.resize(SZ(vec[i]) + SZ(vec[i + 1]));
		merge(ALL(vec[i]), ALL(vec[i + 1]), cur.begin());
		FOR(j, 0, SZ(cur))
		{
			if ((c[i] == c[i + 1]) ^ (a[i] / cur[j] == a[i + 1] / cur[j]) ^ 1)
			{
				events.PB({cur[j], 1});
				if (j + 1 < SZ(cur))
					events.PB({cur[j + 1], -1});
			}
		}
	}
	sort(ALL(events));
	int curBal = 0, ans = 0;
	FOR(i, 0, SZ(events))
	{
		auto [x, d] = events[i];
		if (curBal == n - 1 && i > 0)
			ans += x - events[i - 1].F;
		curBal += d;
	}
	if (curBal == n - 1)
		cout << "-1\n";
	else
		cout << ans << "\n";
	return 0;
}