#include<iostream>

#include<cstdio>

#include<algorithm>

using namespace std;

typedef long long ll;

ll t,a1,b1,a2,b2,x_0,y_0;

ll ex_gcd(ll a,ll b,ll &x,ll &y)

{

    if(!b){x=1,y=0;return a;}

    ll d=ex_gcd(b,a%b,x,y);

    ll tmp=x;

    x=y,y=tmp-a/b*y;

    return d;

}

int main()

{

    while(~scanf("%lld",&t))

    {

        bool ck=1;

        scanf("%lld%lld",&a1,&b1);

        t--;

        while(t--)

        {

            scanf("%lld%lld",&a2,&b2);

            ll a=a1,b=a2,c=b2-b1;

            ll d=ex_gcd(a,b,x_0,y_0);

            if(c%d) ck=0;

            ll t=b/d;

            x_0=(x_0*(c/d)%t+t)%t;

            b1=a1*x_0+b1;

            a1=a1*(a2/d);

        }

        if(!ck) printf("-1\n");

        else printf("%lld\n",b1);

    }

    return 0;

 }