#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
#define FOR(i , a , b) for(int i = (a) , _b = (b); i <= _b; ++i)
#define MASK(x) ((LL)(1) << (x))
#define BIT(mask , x) (((mask)>>(x))&(1))
#define sz(x) (int)(x).size()
template<class T1 , class T2>
bool maximize(T1 &a , T2 b){
if (a < b) return a = b , true; else return false;
}
template<class T1 , class T2>
bool minimize(T1 &a , T2 b){
if (a > b) return a = b , true; else return false;
}
template<class T>
void compress(vector<T>&data){
sort(data.begin() ,data.end());
data.resize(unique(data.begin() , data.end()) - data.begin());
}
template<class T1 , class T2>
T2 Find(const vector<T1>&data , T2 y){
return upper_bound(data.begin() , data.end() , y) - data.begin();
}
const int N = (int) 3e4;
const LL inf = (LL)1e18;
const int MAXBLOCK = 174;
int n , m;
int b[N + 2] , p[N + 2];
vector<int>jump_step[N + 2];
struct Node{
int u;
int step;
LL cost;
bool operator < (const Node&other) const{
return cost > other.cost;
}
};
LL dist[N + 2][MAXBLOCK + 2];
int main(){
ios::sync_with_stdio(false);
cin.tie(0) ; cout.tie(0) ;
#define name "main"
if (fopen(name".inp","r")){
freopen(name".inp","r",stdin);
freopen(name".out","w",stdout);
}
cin >> n >> m;
for(int i = 1; i <= m; ++i){
cin >> b[i] >> p[i];
++b[i];
jump_step[b[i]].push_back(p[i]);
}
for(int i = 1; i <= n; ++i) compress(jump_step[i]);
memset(dist , 0x3f , sizeof dist);
dist[b[1]][0] = 0;
priority_queue<Node> pq;
pq.push({b[1] , 0 , dist[b[1]][0]});
LL ans = inf;
while (sz(pq)){
int u = pq.top().u;
int step = pq.top().step;
LL cost = pq.top().cost;
pq.pop();
if (cost !=dist[u][step]) continue;
if (u == b[2]){
minimize(ans , dist[u][step]);
break;
}
if (step > 0 && u - step > 0 && minimize(dist[u - step][step] , cost + 1)) {
pq.push({u - step , step , dist[u - step][step]});
}
if (step > 0 && u + step <= n && minimize(dist[u + step][step] , cost + 1)){
pq.push({u + step , step , dist[u + step][step]});
}
for(int nxt_step : jump_step[u]){
if (nxt_step >= MAXBLOCK){
for(int j = u + nxt_step , add = 1; j <= n; j += nxt_step , ++add) {
if (minimize(dist[j][0] , cost + add)){
pq.push({j , 0 , dist[j][0]});
}
}
for(int j = u - nxt_step, add = 1; j >= 1; j -= nxt_step , ++add){
if (minimize(dist[j][0] , cost + add)){
pq.push({j , 0 , dist[j][0]});
}
}
}
else {
int nxt_v = u + nxt_step;
int prev_v = u - nxt_step;
if (nxt_v <= n && minimize(dist[nxt_v][nxt_step] , cost + 1)){
pq.push({nxt_v , nxt_step , dist[nxt_v][nxt_step]});
}
if (prev_v > 0 && minimize(dist[prev_v][nxt_step] , cost + 1)){
pq.push({prev_v , nxt_step , dist[prev_v][nxt_step]});
}
}
}
jump_step[u].clear();
}
if (ans == inf) ans = -1;
cout << ans;
return 0;
}
