#include <cstdio>
#include <algorithm>
#include <cstring>
#include <iostream>
#include <vector>
#include <set>
#include <queue>
#include <map>
#include <ctime>
#include <random>
#include <cassert>
#include <numeric>
#include <cmath>
#include <bitset>
#include <ext/pb_ds/assoc_container.hpp>
#define pii pair<int, int>
#define fi first
#define se second
#define MP make_pair
#define ep emplace
#define eb emplace_back
//#define int long long
#define rep(i, j, k) for (int i = (j); i <= (k); i++)
#define per(i, j, k) for (int i = (j); i >= (k); i--)
typedef double db;
typedef long double ldb;
typedef long long ll;
//typedef __int128 lll;
typedef unsigned long long ull;
typedef unsigned int ui;
using namespace std;
using namespace __gnu_pbds;
bool Mbe;

//char buf[1<<20],*p1,*p2;
//#define getchar() (p1 == p2 && (p2 = (p1 = buf) + fread(buf,1,1<<20,stdin), p1 == p2) ? 0 : *p1++)
int read() {
	int s = 0, f = 1;
	char c = getchar();
	while (c < '0' || c > '9') f ^= (c == '-'), c = getchar();
	while (c >= '0' && c <= '9') s = s * 10 + c - '0', c = getchar();
	return f ? s : -s;
}
template<typename T>void chkmax(T&x,const T&y){x=max(x,y);}
template<typename T>void chkmin(T&x,const T&y){x=min(x,y);}

const int N=25;
int n,L,R,a[N],c[N],b[N<<18],tot,ans,id[N];

bool Med;
signed main() {
	fprintf(stderr,"%.3lfMb\n",(&Mbe-&Med)/1024./1024.);
	n=read();
	rep(i,1,n){
		a[i]=read()-1,c[i]=read();
		for(int l=1,r;l<=a[i];l=r+1){
			r=a[i]/(a[i]/l);
			b[++tot]=l;
		}
		b[++tot]=a[i]+1;
	}
	sort(b+1,b+tot+1);
	tot=unique(b+1,b+tot+1)-b-1;
	rep(i,1,tot){
		memset(id,-1,sizeof id);
		bool flag=1;
		rep(j,1,n){
			if(~id[c[j]]){
				if(a[j]/b[i]!=id[c[j]])flag=0;
			}else id[c[j]]=a[j]/b[i];
		}
		rep(j,1,n-1)if(id[c[j]]==id[c[j+1]]&&c[j]!=c[j+1])flag=0;
		if(flag){
			if(i==tot)printf("-1\n"),exit(0);
			else ans+=b[i+1]-b[i];
		}
	}
	printf("%d\n",ans);
	return 0;
}