#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 int inf = 1e9 + 5;
const int mxN = 30005;
int n, m;
int B[mxN];
int P[mxN];
vector<pii> g[mxN];
int dist[mxN];
unordered_map<int,int> mp[mxN];
int id[mxN];
bool cmp(int s1, int s2){
return B[s1] < B[s2];
}
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)id[i] = i;
sort(id, id + n, cmp);
ff(i,0,n - 1){
if(id[i] == 1)continue;
int pos = B[id[i]] + P[id[i]], cost = 1;
while(pos < m){
g[B[id[i]]].pb({pos, cost}); cost += 1;
if(mp[pos].count(P[id[i]]) == 1)break;
mp[pos][P[id[i]]] = 1;
pos += P[id[i]];
}
}
ff(i,0,m - 1)mp[i].clear();
ff(i,0,n - 1){
if(id[i] == 1)continue;
int pos = B[id[i]] - P[id[i]], cost = 1;
while(pos >= 0){
g[B[id[i]]].pb({pos, cost}); cost += 1;
if(mp[pos].count(P[id[i]]) == 1)break;
mp[pos][P[id[i]]] = 1;
pos -= P[id[i]];
}
}
ff(i,0,m - 1)dist[i] = inf;
priority_queue<pii, vector<pii>, greater<pii>> pq;
pq.push({dist[S] = 0, S});
while(sz(pq)){
pii 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;
}
/*
6 4
4 5
0 3
5 1
4 1
18 10
1 2
0 12
1 17
4 1
3 1
5 13
14 7
5 16
15 6
9 5
51 9
9 1
45 2
42 38
17 9
39 44
35 1
22 41
43 35
3 36
22 19
10 10
13 5
1 1
17 17
2 3
3 9
20 22
18 1
3 5
11 16
7 16
2 22
20 4
11 19
7 1
16 1
11 14
8 9
3 18
5 3
0 2
1 1
4 1
// probati bojenje sahovski
*/
