#include <bits/stdc++.h>
/*#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/detail/standard_policies.hpp>*/
#define pb push_back
#define mp make_pair
#define sz(s) ((int)(s.size()))
#define all(s) s.begin(), s.end()
#define rep(i, a, n) for (int i = a; i <= n; ++i)
#define per(i, n, a) for (int i = n; i >= a; --i)
#define onlycin ios_base::sync_with_stdio(false); cin.tie(0)
#define F first
#define S second
using namespace std;
// using namespace __gnu_pbds;
typedef long long ll;
typedef unsigned long long ull;
/*typedef tree<
pair < int, int >,
null_type,
less< pair < int, int > >,
rb_tree_tag,
tree_order_statistics_node_update>
ordered_set;*/
// find_by_order() order_of_key()
const int MAXN = (int)5e5+228;
const char nxtl = '\n';
const int mod = (int)1e9+7;
const double eps = (double)1e-7;
template<typename T> inline bool updmin(T &a, const T &b) {return a > b ? a = b, 1 : 0;}
template<typename T> inline bool updmax(T &a, const T &b) {return a < b ? a = b, 1 : 0;}
int n, m, b[MAXN], p[MAXN], d[2020];
vector < pair < int, int > > g[2020];
vector < int > divs[2020], have[2020];
int main() {
#ifdef accepted
freopen(".in", "r", stdin);
freopen(".out", "w", stdout);
#endif
onlycin;
cin >> n >> m;
rep(i, 1, m) {
cin >> b[i] >> p[i];
have[++b[i]].pb(p[i]);
}
rep(i, 1, n) {
sort(all(have[i])); have[i].resize(unique(all(have[i]))-have[i].begin());
}
memset(d, 0x3f, sizeof d);
d[b[1]] = 0;
set < pair < int, int > > S; S.insert(mp(0, b[1]));
while(!S.empty()) {
int v = S.begin()->second;
S.erase(S.begin());
for(auto &to : have[v]) {
int cnt = 0;
for(int go = v + to; go <= n; go += to) {
cnt++;
if(cnt + d[v] < d[go]) {
S.erase(mp(d[go], go));
d[go] = d[v]+cnt;
S.insert(mp(d[go], go));
}
if(binary_search(all(have[go]), to)) break;
}
cnt = 0;
for(int go = v - to; go > 0; go -= to) {
cnt++;
if(cnt + d[v] < d[go]) {
S.erase(mp(d[go], go));
d[go] = d[v]+cnt;
S.insert(mp(d[go], go));
}
if(binary_search(all(have[go]), to)) break;
}
}
}
if(d[b[2]] == d[0]) d[b[2]] = -1;
cout << d[b[2]] << nxtl;
return 0;
}
