#define wiwihorz
#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma loop-opt(on)
#define rep(i, a, b) for(int i = a; i <= b; i ++)
#define rrep(i, a, b) for(int i = b; i >= a; i --)
#define all(x) x.begin(), x.end()
#define ceil(a, b) ((a + b - 1) / (b))
#define INF 1000000000
using namespace std;
#define pii pair<int, int>
namespace solver {
const int P = 4000000;
const int K = 200;
int n, m, st, ed, ii;
vector<int> dis, s;
vector<pair<int, bool>> mp[P];
vector<int> a[K][K];
void init_(int _n, int _m) {
n = _n, m = _m, ii = n - 1;
s.clear();
dis.assign(P, INF);
}
void get_new() {
ii++;
if(ii >= P) exit(0);
}
void add_node(int pos, int b) {
for(int i = pos % b; i < n; i += b) {
get_new();
if(i == pos) mp[i].push_back({ii, 0});
if(i != pos % b) {
mp[ii].push_back({ii - 1, 1});
mp[ii - 1].push_back({ii, 1});
}
mp[ii].push_back({i, 0});
}
}
int solve() {
rep(i, 1, K - 1) rep(j, 0, i - 1) {
if(!a[i][j].size()) continue;
sort(all(a[i][j]));
vector<int> &v = a[i][j];
int id = -1;
for(int k = j; k < n; k += i) {
get_new();
while(id + 1 < v.size() && v[id + 1] <= k) id ++;
if(id >= 0 && v[id] == k) mp[k].push_back({ii, 0});
if(k != j) {
mp[ii].push_back({ii - 1, 1});
mp[ii - 1].push_back({ii, 1});
}
mp[ii].push_back({k, 0});
}
}
dis[st] = 0;
deque<int> q;
q.push_back(st);
while(q.size()) {
int cur = q.front();
q.pop_front();
for(auto i : mp[cur]) if(dis[i.first] == INF) {
dis[i.first] = i.second + dis[cur];
if(i.second) q.push_back(i.first);
else q.push_front(i.first);
if(i.first == ed) return dis[ed];
}
}
return (dis[ed] == INF ? -1 : dis[ed]);
}
};
using namespace solver;
signed main() {
ios::sync_with_stdio(false), cin.tie(0);
int n, m, b, p;
cin >> n >> m;
init_(n, m);
rep(i, 0, m - 1) {
cin >> b >> p;
if(i == 0) st = b;
if(i == 1) ed = b;
if(p >= K) add_node(b, p);
else a[p][b % p].push_back(b);
}
cout << solve() << "\n";
return 0;
}
