#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <bits/stdc++.h>
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef pair<ld,ld> pdd;
#define ff(i,a,b) for(int i = a; i <= b; i++)
#define fb(i,b,a) for(int i = b; i >= a; i--)
#define trav(a,x) for (auto& a : x)
#define sz(a) (int)(a).size()
#define pb push_back
#define fi first
#define se second
#define lb lower_bound
#define ub upper_bound
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
// os.order_of_key(k) the number of elements in the os less than k
// *os.find_by_order(k) print the k-th smallest number in os(0-based)
const int mod = 1000000007;
const ll inf = 1e15 + 5;
const int mxN = 30005;
int n, m;
int B[mxN];
int P[mxN];
vector<pii> g[mxN];
ll dist[mxN];
map<int,int> mp[mxN];
int main() {
cin.tie(0)->sync_with_stdio(0);
cin >> m >> n;
ff(i,0,n - 1)cin >> B[i] >> P[i];
int S = B[0];
int T = B[1];
ff(i,0,n - 1){
int pos = B[i] + P[i], cost = 1;
while(pos < m){
g[B[i]].pb({pos, cost}); cost += 1;
if(mp[pos].count(P[i]) == 1)break;
mp[pos][P[i]] = 1;
pos += P[i];
}
}
ff(i,0,m - 1)mp[i].clear();
ff(i,0,n - 1){
int pos = B[i] - P[i], cost = 1;
while(pos >= 0){
g[B[i]].pb({pos, cost}); cost += 1;
if(mp[pos].count(P[i]) == 1)break;
mp[pos][P[i]] = 1;
pos -= P[i];
}
}
ff(i,0,m - 1)dist[i] = inf;
priority_queue<pll, vector<pll>, greater<pll>> pq;
pq.push({dist[S] = 0, S});
while(sz(pq)){
pll v = pq.top(); pq.pop();
if(dist[v.se] < v.fi)continue;
for(auto c : g[v.se]){
int u = c.fi;
int w = c.se;
if(dist[u] > v.fi + w){
pq.push({dist[u] = v.fi + w, u});
}
}
}
cout << (dist[T] == inf ? -1 : dist[T]) << '\n';
return 0;
}
/*
5 3
0 2
1 1
4 1
// probati bojenje sahovski
*/
