#include <cstring>

#include <cstdio>

#include <algorithm>

#include <cstdlib>

#include <cmath>

using namespace std;

typedef long long LL;

const int N=100005;

LL ex_gcd(LL a,LL b,LL &x,LL &y){

	if (!b){

		x=1,y=0;

		return a;

	}

	LL ans=ex_gcd(b,a%b,y,x);

	y-=(a/b)*x;

	return ans;

}

LL m,a[N],n[N];

LL solve(){

	LL a1,a2,n1,n2,c,d,k1,k2,K,t;

	a1=a[1],n1=n[1];

	for (int i=2;i<=m;i++){

		a2=a[i],n2=n[i],d=ex_gcd(n1,n2,k1,k2),c=a2-a1;

		if (c%d)

			return -1;

		K=c/d*k1,t=n2/d,K=(K%t+t)%t,a1+=n1*K,n1=n1/d*n2;

	}

	return a1;

}

int main(){

	while (~scanf("%lld",&m)){

		for (int i=1;i<=m;i++)

			scanf("%lld%lld",&n[i],&a[i]);

		printf("%lld\n",solve());

	}

	return 0;

}