#include <fstream>
using namespace std;
ifstream input("HanXin.in");
ofstream output("HanXin.out");
long long m,n,s=1,chu[12],yu[12],z=0;
int main()
{
	long long i;
	input>>m>>n;
	for(i=1;i<=n;i++)
	{
		input>>chu[i]>>yu[i];
		s*=chu[i];
	}
	for(i=1;i<=n;i++)
	{
		z=(z+(s/chu[i]%chu[i])*(s/chu[i])*yu[i])%s;
	}
	if(z>=m)
	{
		if(z>m) output<<"-1"<<endl;
		else output<<m<<endl;
	}
	else
	{
		long long res=0;
		res=m-s*(m/s)+s-z;
		output<<res%s<<endl;
	}
	input.close();
	output.close();
	return 0;
}