#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<pair<int,int>> vpi;
#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define F0R(i,a) FOR(i,0,a)
#define ROF(i,a,b) for (int i = (b)-1; i >= (a); --i)
#define R0F(i,a) ROF(i,0,a)
#define trav(a,x) for (auto& a: x)
#define pb push_back
#define rsz resize
#define sz(x) int(x.size())
#define all(x) begin(x), end(x)
#define f first
#define s second
int const n=3e4;
int dist[n];
vpi adj[n];
void dij(int x)
{
dist[x]=0;
priority_queue<pii>q;
bitset<n> pro;
q.push({0,x});
int b,w;
while(!q.empty())
{
int a = q.top().s;
q.pop();
if(pro[a])continue;
pro[a]=1;
trav(e, adj[a])
{
b=e.f;
w=e.s;
if(dist[a]+w<dist[b])
{
dist[b]=dist[a]+w;
q.push({-dist[b],b});
}
}
}
}
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0);
int N,M;
cin>>N>>M;
int const m=3e4;
int B[m], P[m];
vi powin[n];
F0R(i,M)
{
cin>>B[i]>>P[i];
powin[B[i]].pb(P[i]);
}
fill(dist,dist+n,1e9);
F0R(i,N)
{
trav(pow, powin[i])
{
for(int j=0; i+j*pow<N; ++j)
{
adj[i].pb({i+j*pow, j});
}
for(int j=-1; i+j*pow>=0; --j)
{
adj[i].pb({i+j*pow, -j});
}
}
}
dij(B[0]);
if(dist[B[1]]==1e9)cout<<-1<<'\n';
else cout<<dist[B[1]]<<'\n';
return 0;
}
