#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <queue>
#include <algorithm>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <memory.h>
#include <ctime>
#include <bitset>
#include <unordered_map>

using namespace std;

#define ABS(a) ((a>0)?a:-(a))
#define MIN(a,b) ((a<b)?(a):(b))
#define MAX(a,b) ((a<b)?(b):(a))
#define FOR(i,a,n) for (int i=(a);i<(n);++i)
#define FI(i,n) for (int i=0; i<(n); ++i)
#define pnt pair <int, int>
#define mp make_pair
#define PI 3.1415926535897
#define MEMS(a,b) memset(a,b,sizeof(a))
#define LL long long
#define U unsigned

int used[30];
set<int> s;
int a[30];
int c[30];

const int INF = 1000000001;

vector<pair<int,pnt > > get1(int n)
{
	vector<pair<int,pnt > > res;
	int i = 1;
	for (i = 1; i*i<=n; ++i)
	{
		res.push_back(mp(n/i,mp(i,i)));
	}
	while (i<=n)
	{
		int k = n/i;
		int to = n/k;
		res.push_back(mp(k,mp(i,to)));
		i = to + 1;
	}
	res.push_back(mp(0,mp(n+1,INF)));
	reverse(res.begin(), res.end());
	return res;
}

vector<pnt > all;

void get2(int a, int b, int v)
{
	vector<pair<int,pnt > > res1 = get1(a);
	vector<pair<int,pnt > > res2 = get1(b);
	int i = 0, j = 0;
	while ((i<res1.size()) && (j<res2.size()))
	{
		if (res1[i].first == res2[j].first)
		{
			int l = max(res1[i].second.first, res2[j].second.first);
			int r = min(res1[i].second.second, res2[j].second.second);
			if (l<=r)
			{
				//cout<<l<<" "<<r<<endl;
				all.push_back(mp(l,v));
				all.push_back(mp(r+1,-v));
			}
			i++;
			j++;
		}
		else
			if (res1[i].first<res2[j].first)
				i++;
			else
				j++;
	}
	//cout<<endl;
}

vector<int> order;

int min1[30];
int max1[30];

void brut(int x, int y)
{
	FOR(i,1,y+2)
	{
		if (x/i == y/i)
			cout<<i<<endl;
	}
}

int main() {
#ifdef Fcdkbear
	freopen("in.txt", "r", stdin);
	double beg = clock();
	//freopen("out.txt", "w", stdout);
#endif

	/*brut(2000,2015);
	return 0;*/
	/*vector<pair<int, pnt > > t = get1(25);
	FOR(i,0,t.size())
	{
		cout<<t[i].first<<": "<<t[i].second.first<<" "<<t[i].second.second<<endl;
	}
	return 0;*/
	int n;
	cin>>n;
	FOR(i,0,n)
	{
		cin>>a[i]>>c[i];
		a[i]--;
		s.insert(c[i]);
		if ((order.size() == 0) || (order.back() != c[i])) {
			order.push_back(c[i]);
			min1[c[i]] = a[i];
		}
		max1[c[i]] = a[i];
	}
	if (s.size() == 1)
	{
		cout<<-1<<endl;
		return 0;
	}
	used[c[0]]=1;
	FOR(i,1,n)
	{
		if ((c[i]!=c[i-1]) && (used[c[i]]))
		{
			cout<<0<<endl;
			return 0;
		}
	}
	int tot = 0;
	FOR(i,0,n)
		FOR(j,i,n)
			if (c[i] == c[j])
			{
				tot++;
				get2(a[i],a[j],1);
			}
			else
				get2(a[i],a[j],-1);
	sort(all.begin(), all.end());
	int last = 0;
	int res = 0;
	int p = 0;
	int cnt = 0;
	while (p<all.size())
	{
		if (all[p].first > 1000000000)
			break;
		int c = p;
		while ((c<all.size()) && (all[c].first == all[p].first))
		{
			cnt+=all[c].second;
			c++;
		}
		if (cnt == tot)
		{
			//cout<<all[p].first << " " << all[c].first - 1<<endl;
			res += all[c].first - all[p].first;
		}
		p = c;
	}
	cout<<res<<endl;

#ifdef Fcdkbear
	double end = clock();
	fprintf(stderr, "*** Total time = %.3lf ***\n",
			(end - beg) / CLOCKS_PER_SEC);
#endif
	return 0;
}