#include<cstdio>
using namespace std;
long long a[15], p[15];
void exgcd(long long a, long long b, long long &x, long long &y){
	if (!b){
		x = 1; y = 0;
		return;
	}
	exgcd(b, a % b, x, y);
	long long t(x);
	x = y; y = t - a / b * y;
}
int main(){
	freopen("HanXin.in", "r", stdin);
	freopen("HanXin.out", "w", stdout);
	long long n, x(0), s(1), b, t, si;
	int m, i;
	scanf("%lld%d", &n, &m);
	for (i = 0; i < m; ++i){
		scanf("%lld%lld", &p[i], &a[i]);
		s *= p[i];
	}
	for (i = 0; i < m; ++i){
		exgcd(si = s / p[i], p[i], b, t);
		x += si * b * a[i] % s;
	}
	x = (n - x) % s;
	if (x < 0) x += s;
	if (x > n) puts("-1");
	else if (x == n){
		for (i = 0; i < m; ++i) if (a[i]) break;
		printf("%lld\n", i == m? x: -1ll);
	}
	else printf("%lld\n", x);
	return 0;
}