#include <bits/stdc++.h>
using namespace std;
#define in ({int x=0;int c=getchar(),n=0;for(;!isdigit(c);c=getchar()) n=(c=='-');for(;isdigit(c);c=getchar()) x=x*10+c-'0';n?-x:x;})
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int rnd(int l,int r){return l+rng()%(r-l+1);}
#define fasty ios_base::sync_with_stdio(false),cin.tie(nullptr);
#define task "SKYSCRAPER"
#define forinc(a,b,c) for(int a=b,_c=c;a<=_c;++a)
#define fordec(a,b,c) for(int a=b,_c=c;a>=_c;--a)
#define forv(a,b) for(auto&a:b)
#define fi first
#define se second
#define pb push_back
#define ii pair<int,int>
#define mt make_tuple
#define all(a) a.begin(),a.end()
#define reset(f, x) memset(f, x, sizeof(f))
#define bit(x,i) ((x>>(i-1))&1)
#define on(x,i) (x|(1ll<<(i-1)))
#define off(x,i) (x&~(1<<(i-1)))
const int N=30010;
int n,m;
int b[N],p[N],f[N];
priority_queue<ii,vector<ii>,greater<ii>> q;
vector<int> val[N];
main()
{
//freopen(task".inp","r",stdin);
//freopen(task".out","w",stdout);
n=in,m=in;
forinc(i,1,m) b[i]=in,p[i]=in,val[b[i]].pb(p[i]);
reset(f,127); f[b[1]]=0;
q.push({0,b[1]});
while(q.size())
{
ii u=q.top(); q.pop();
if(u.fi!=f[u.se]) continue;
forv(i,val[u.se])
{
int c=u.fi;
for(int j=u.se+i;j<n;j+=i) if(f[j]>++c) q.push({f[j]=c,j});
c=u.fi;
for(int j=u.se-i;j>=0;j-=i) if(f[j]>++c) q.push({f[j]=c,j});
}
}
cout<<(f[b[2]]<1e9 ? f[b[2]] : -1);
}
