/*
author: CRISTIANO RONALDO
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
typedef tree<int,null_type,less<int>,rb_tree_tag,
tree_order_statistics_node_update> indexed_set;
can do: find_by_order, order_of_key
__builtin_clz(x): the number of zeros at the beginning of the number
__builtin_ctz(x): the number of zeros at the end of the number
__builtin_popcount(x): the number of ones in the number
__builtin_parity(x): the parity (even or odd) of the number of ones
mt19937 rng(15);
mt19937 bruh(chrono::steady_clock::now().time_since_epoch().count());
mt19937_64 rofl(chrono::steady_clock::now().time_since_epoch().count());
*/
#include <bits/stdc++.h>
#define ll long long
#define all(a) a.begin(), a.end()
#define F first
#define S second
#define pb push_back
#define ull unsigned long long
#define ld long double
#define lv v+v
#define rv v+v+1
#define files freopen("aa.txt", "r", stdin), freopen("aa.txt", "w", stdout)
#define pll pair <long long, long long>
#define pii pair <int, int>
using namespace std;
long long mod = 1e9 + 7;
const ll N = 3e4 + 10;
const ll M = 2e3 + 10;
const ll P = 337ll;
const ld EPS = 1e-7;
const ll block = 325;
const ll inf = 2e18;
ll n,m,b[N],p[N], d[30010];
vector <ll> road[30010];
// v[j] - куда может прыгнуть собака j
// cost[j][u] - цена прыжка собаки j в точку u
// road[j] - Какие собаки живут в точке j
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
cin>>n>>m;
for(ll i = 0; i<m; i++) {
cin>>b[i]>>p[i];
d[i] = inf;
road[b[i]].pb(i);
}
set <pll> q;
q.insert({0,0});
d[0] = 0;
ll ver, ttt, j;
while(!q.empty()) {
ver = (*q.begin()).S; ttt = (*q.begin()).F; q.erase(q.begin());
if(ttt > d[ver]) {continue;}
j = ver;
for(ll i = 1; b[j] + i * p[j] < n; i += 1) {
if(!road[b[j] + i * p[j]].empty()) {
for(auto jj : road[b[j] + i * p[j]]) {
if(d[jj] > d[ver] + i) {
d[jj] = d[ver] + i;
q.insert({d[jj], jj});
}
}
}
}
for(ll i = 0; b[j] - i * p[j] >= 0; i += 1) {
if(!road[b[j] - i * p[j]].empty()) {
for(auto jj : road[b[j] - i * p[j]]) {
if(d[jj] > d[ver] + i) {
d[jj] = d[ver] + i;
q.insert({d[jj], jj});
}
}
}
}
}
cout<<(d[1] == inf ? -1 : d[1]);
return 0;
}
// equal, min, max, 1, random, build
/*
*/
