#include<bits/stdc++.h>
using namespace std;
#define ll int
#define ss second
#define ff first
#define INF 2000000000000000
#define pb push_back
#define edge pair<ll, pair<ll,ll> > // une, number ,power
#define cost first
#define num second.first
#define pwr second.second
ll n,m,l,r,i,j,ii,jj,k,x,y,D[30005][180],s,e;
vector< edge > v[30005][180]; // number , power (powergu oroi bol power = 179)
int main(){
cin.tie(nullptr);
cout.tie(nullptr);
ios::sync_with_stdio(false);
cin>>n>>m;
for(i=1;i<=m;i++){
cin>>l>>r;
if(i==1) s=l;
if(i==2) e=l;
if(r>175)
for(j=1 ; l+r*j<n || l-r*j>=0 ; j++){
edge a;
a.num=l+r*j;
a.pwr=179;
a.cost=j;
if(a.num<n) v[l][179].pb(a);
a.num=l-r*j;
if(a.num>=0) v[l][179].pb(a);
}
else{
v[l][179].pb({0,{l,r}});
}
}
for(i=1;i<=175;i++)
for(j=0;j<n;j++){
v[j][i].pb({0,{j,179}});
if(j+i<n) v[j][i].pb({1,{j+i,i}});
if(j-i>=0) v[j][i].pb({1,{j-i,i}});
}
priority_queue<edge , vector< edge >, greater< edge > > q;
memset(D,-1,sizeof(D));
q.push({0,{s,179}});
while(!q.empty()){
ll une, nmbr, power;
une=q.top().cost;
nmbr=q.top().num;
power=q.top().pwr;
q.pop();
if(D[nmbr][power]!=-1) continue;
D[nmbr][power]=une;
for(edge no : v[nmbr][power]) if(D[no.num][no.pwr]==-1) q.push({une+no.cost,{no.num,no.pwr}});
}
cout<<D[e][179];
}
