#include<iostream>#include<algorithm>using namespace std;typedef long long ll;ll exgcd( ll a, ll b, ll& x, ll& y ){if(b == 0){x = 1;y = 0;return a;}ll gcd = exgcd( b, a%b, x, y );ll tem = x;x = y;y = tem - a / b*y;return gcd;}int main(){int n;ll r1, m1, r2, m2, x0, y0;while(cin >> n){bool flag=1;cin >> m1 >> r1;for(int i = 1; i < n; i++){cin >> m2 >> r2;ll a = m1, b = m2, c = r2 - r1;ll d=exgcd( a, b, x0, y0 );if(c%d != 0) flag = false;ll t = b / d;x0 = (x0*(c / d) % t + t) % t;r1 = m1*x0 + r1;m1 = m1*(m2 / d);}if(!flag){cout << -1 << endl;continue;}cout << r1 << endl;}return 0;}
