#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 };
int n,m;
int B[30001],P[30001];
map<Pi,int> M;
Vi t[30001];
VPi vv;
vector<Vi> v;
bool chk[30001];
Vi V[30001];
int main() {
scanf("%d%d",&n,&m);
fup(i,0,m-1,1)
{
scanf("%d%d",B+i,P+i);
t[P[i]].pb(B[i]%P[i]);
}
fup(i,1,30000,1)
{
sort(ALL(t[i]));t[i].resize(unique(ALL(t[i]))-t[i].begin());
for(int x:t[i])
{
fup(j,x,n-1,i)
{
vv.pb(mp(j,i));
M[mp(j,i)]=vv.size()-1;
}
}
}
int N=vv.size();
v.resize(N);
fup(i,1,N-1,1)
if(vv[i].Y==vv[i-1].Y && vv[i].X%vv[i].Y==vv[i-1].X%vv[i].Y)
{
v[i].pb(i-1);
v[i-1].pb(i);
}
fup(i,0,m-1,1)
{
V[B[i]].pb(M[mp(B[i],P[i])]);
}
Vi dist(N,-1);
deque<int> q;
int S,E;
fup(i,0,N-1,1)
{
if(mp(B[0],P[0])==vv[i])S=i;
if(mp(B[1],P[1])==vv[i])E=i;
}
q.pb(S);
dist[S]=0;
while(!q.empty())
{
int x=q.front();
q.pop_front();
for(int y:v[x])
{
if(dist[y]==-1)
{
dist[y]=dist[x]+1;
q.pb(y);
}
}
if(!chk[vv[x].X])
{
chk[vv[x].X]=1;
for(int z:V[vv[x].X])
{
dist[z]=dist[x];
q.push_front(z);
}
}
}
printf("%d",dist[E]);
}
