#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
typedef long long ll;
typedef double db;
typedef vector<long long> vl;
typedef pair<long long, long long > pl;
const int N = 1e6 + 1;
#define po pop_back
#define pb push_back
#define mk make_pair
#define lw lower_bound
#define up upper_bound
#define ff first
#define ss second
#define boost ios_base::sync_with_stdio(); cin.tie(0); cout.tie(0);
#define MOD 1000000007
#define MAX 1e18
#define MIN -1e18
#define per(i,a,b) for(ll i=b;i>=a;i--)
#define con continue
#define freopen freopen("input.txt", "r", stdin);freopen("output.txt", "w", stdout);
#define PI 3.14159265358979323846264338327950288419716939937510582097494459230781640628
// typedef tree<ll , null_type, less<ll>, rb_tree_tag, tree_order_statistics_node_update> indexed_set;
// template< typename T>
// using indexed_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
ll n, m, mid, res,pos, mn, mx, sum, h1, h2, arr[3234567],arr1[1234567],k, i, j, h, a, x, y, z,par[1234567];
bool used[1234567];
ll tp[1234567];
ll dx[4]={-1,1,0,0},dy[4]={0,0,-1,1},c1[123][123];
//ll jump[22][223456];
//ll lvl[1234567];
//ll bit[1234567];
//ll timer;
//ll st[1234567],endd[1234567];
//ll dp[5005][5005];
vl dog[1234567];
ll dist[1234567];
priority_queue<pl> pq;
void go(ll node){
used[node]=1;
for(auto u:dog[node]){
for(int i=node+u,a=1;i<n;i+=u,a++){
if(dist[i] > dist[node] + a){
dist[i] = dist[node]+a;
pq.push({-dist[i] , i});
}
}
for(int i=node-u,a=1;i>=0;i-=u,a++){
if(dist[i] > dist[node] + a){
dist[i] = dist[node]+a;
pq.push({-dist[i] , i});
}
}
}
}
class A {
public :
static void funcA() {
cin>>n>>m;
ll st,en,b,p;
cin>>st>>p;
dog[st] . pb(p);
cin>>en>>p;
dog[en] . pb(p);
m-=2;
while(m--){
cin>>b>>p;
dog[b].pb(p);
}
for(int i=0;i<=1e6;i++)dist[i]=MAX;
pq.push({0,st});
dist[st]=0;
while(pq.size()>0){
ll a = pq.top().ss;
pq.pop();
if(a==en){
cout<<dist[en];
exit(0);
}
if(used[a]==0){
go(a);
}
}
cout<<-1;
}
}lol;
void solve() {
A::funcA();
}
int main() {
ll T = 1;
//cin>>T;
boost;
while (T--) {
solve();
}
}
