//by tsingyawn
#include<cstdio>
#include<ctime>
#include<iostream>
#include<cstring>
#include<algorithm>
using namespace std;

typedef long long LL;

const int maxm=15;

LL n,m,p[maxm],a[maxm];

void init()
{
	cin>>n>>m;
	for(int i=0;i<m;i++) cin>>p[i]>>a[i];
}

void work()
{
	if(n>1000000000){
		printf("-1\n");
		return;
	}
	for(LL i=n;i>=0;i--){
		bool ok=true;
		for(int j=0;j<m;j++) if(i%p[j]!=a[j]){
			ok=false;
			break;
		}
		if(ok){
			cout<<n-i<<endl;
			return;
		}
		if(clock()/(double)CLOCKS_PER_SEC>=0.93){
			printf("-1\n");
			return;
		}
	}
}

int main()
{
	freopen("HanXin.in","r",stdin);
	freopen("HanXin.out","w",stdout);
	init();
	work();
	fclose(stdin);
	fclose(stdout);
	return 0;
}