#include<iostream>  
#include<cstdio>
#include<cmath>
#include<cstring>  
#include<algorithm>  
using namespace std; 
long long ans=0,l,n,up=1,a[11],b[11];
int main()
{ 
    freopen("HanXin.in","r",stdin);        
    freopen("HanXin.out","w",stdout);      
	cin>>l>>n;
	for(int i=1;i<=n;i++)cin>>a[i]>>b[i];
	for(int i=1;i<=n;i++)
	{
		
	    int p=0;
		while(p==0)
		{
		    if((l-ans)%a[i]==b[i])
			{
			    p=1;
				up*=a[i];
			}
			else
			{
			    ans+=up;
				if(ans>=l)
				{
					p=2;
					break;
				}
			}
			if(p==2)break;
		}
	}
	if(ans>=l)
	{
	    cout<<"-1";
		return 0;
	}
	cout<<ans;
    return 0;  
}