#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 fi first
#define se second
int const M=3e4+3;
int b[M],p[M];
int const N=3e4+3;
vpi adj[N];
bitset<N> vis;
int d[N];
int const INF=1e9;
vi doges[N];
vi visited;
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0);
int n,m;
cin>>n>>m;
F0R(i,m)
{
cin>>b[i]>>p[i];
doges[b[i]].pb(i);
}
int mxjump=0;
priority_queue<pii> q;
q.push({0,b[0]});
fill(d,d+n,INF);
d[b[0]]=0;
while(!q.empty() && (!vis[b[1]]))
{
int u=q.top().se;
q.pop();
if(vis[u])continue;
vis[u]=1;
visited.pb(u);
if(d[u]==mxjump)
{
++mxjump;
trav(e,visited)
{
trav(doge,doges[e])
{
int cand = d[e]+mxjump;
int nxt = e+p[doge]*mxjump;
if(nxt<n && cand < d[nxt])
{
d[nxt] = cand;
q.push({-cand,nxt});
}
nxt = e-p[doge]*mxjump;
if(nxt>=0 && cand < d[nxt])
{
d[nxt] = cand;
q.push({-cand,nxt});
}
}
}
}
}
if(d[b[1]]==INF)cout<<-1<<'\n';
else cout<<d[b[1]]<<'\n';
return 0;
}
