#include<cstdio>
#include<cstdlib>
#include<iostream>
#include<vector>
#include<queue>
#include<bitset>
#include<string>
#include<stack>
#include<set>
#include<unordered_set>
#include<map>
#include<unordered_map>
#include<cstring>
#include<complex>
#include<cmath>
#include<iomanip>
#include<numeric>
#include<algorithm>
#include<list>
#include<functional>
#include<cassert>
#define mp make_pair
#define pb push_back
#define X first
#define Y second
#define y0 y12
#define y1 y22
#define INF 987654321
#define PI 3.141592653589793238462643383279502884
#define fup(i,a,b,c) for(int (i)=(a);(i)<=(b);(i)+=(c))
#define fdn(i,a,b,c) for(int (i)=(a);(i)>=(b);(i)-=(c))
#define MEM0(a) memset((a),0,sizeof(a));
#define MEM_1(a) memset((a),-1,sizeof(a));
#define ALL(a) a.begin(),a.end()
#define SYNC ios_base::sync_with_stdio(false);cin.tie(0)
using namespace std;
typedef long long ll;
typedef long double ld;
typedef double db;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef pair<int, int> Pi;
typedef pair<ll, ll> Pll;
typedef pair<ld, ld> Pd;
typedef vector<int> Vi;
typedef vector<ll> Vll;
typedef vector<double> Vd;
typedef vector<Pi> VPi;
typedef vector<Pll> VPll;
typedef vector<Pd> VPd;
typedef tuple<int, int, int> iii;
typedef tuple<int,int,int,int> iiii;
typedef tuple<ll, ll, ll> LLL;
typedef vector<iii> Viii;
typedef vector<LLL> VLLL;
typedef complex<double> base;
const ll MOD = 1000000007;
ll POW(ll a, ll b, ll MMM = MOD) {ll ret=1; for(;b;b>>=1,a=(a*a)%MMM)if(b&1)ret=(ret*a)% MMM; return ret; }
ll gcd(ll a, ll b) { return b ? gcd(b, a%b) : a; }
ll lcm(ll a, ll b) { if (a == 0 || b == 0)return a + b; return a*(b / gcd(a, b)); }
int dx[] = { 0,1,0,-1,1,1,-1,-1 }, dy[] = { 1,0,-1,0,1,-1,1,-1 };
set<int> S[30001];
VPi v[30001];
bool chk[30001];
int dist[30001];
int B[30001],P[30001];
int n,m;
int main() {
fill(dist,dist+30001,INF);
scanf("%d%d",&n,&m);
fup(i,0,m-1,1)
{
scanf("%d%d",B+i,P+i);
S[B[i]].insert(P[i]);
}
fup(i,0,n-1,1)
{
for(int p:S[i])
{
int d=0,now=i;
while(1)
{
d++,now+=p;
if(now>=n)break;
v[i].pb(mp(now,d));
if(S[now].find(p)!=S[now].end())break;
}
d=0,now=i;
while(1)
{
d++,now-=p;
if(now<0)break;
v[i].pb(mp(now,d));
if(S[now].find(p)!=S[now].end())break;
}
}
}
priority_queue<Pi> q;
int S=B[0],E=B[1];
q.push({0,S});
dist[S] = 0;
while (!q.empty())
{
int x = q.top().second;
q.pop();
if (chk[x])continue;
chk[x] = 1;
for (int i = 0; i < v[x].size(); i++)
{
int next = v[x][i].first;
int cost = v[x][i].second;
if (dist[next] > dist[x] + cost)
{
dist[next] = dist[x] + cost;
q.push({-dist[next], next});
}
}
}
int ans=dist[E];
if(ans==INF)ans=-1;
printf("%d",ans);
}
