/*
_____
.' '.
/ 0 0 \
| ^ |
| \ / |
\ '---' /
'._____.'
*/
#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")
#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/rope>
using namespace std;
using namespace __gnu_pbds;
using namespace __gnu_cxx;
random_device(rd);
mt19937 rng(rd());
const long long FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
struct custom_hash
{
template<class T>
unsigned long long operator()(T v) const
{
unsigned long long x = v;
x += FIXED_RANDOM; x += 11400714819323198485ull;
x = (x ^ (x >> 30)) * 13787848793156543929ull;
x = (x ^ (x >> 27)) * 10723151780598845931ull;
return x ^ (x >> 31);
}
};
template<class T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<class T, class U> using hash_table = gp_hash_table<T, U, custom_hash>;
template<class T>
void readi(T &x)
{
x = 0;
bool negative = false;
char c = ' ';
while (c < '-')
{
c = getchar();
}
if (c == '-')
{
negative = true;
c = getchar();
}
while (c >= '0')
{
x = x * 10 + (c - '0');
c = getchar();
}
if (negative)
{
x = -x;
}
}
template<class T>
void printi(T output)
{
if (output == 0)
{
putchar('0');
return;
}
if (output < 0)
{
putchar('-');
output = -output;
}
int buf[20], n = 0;
while(output)
{
buf[n] = ((output % 10));
output /= 10;
n++;
}
for (n--; n >= 0; n--)
{
putchar(buf[n] + '0');
}
return;
}
template<class T>
void ckmin(T &a, T b)
{
a = min(a, b);
}
template<class T>
void ckmax(T &a, T b)
{
a = max(a, b);
}
long long expo(long long a, long long e, long long mod)
{
return ((e == 0) ? 1 : ((expo(a * a % mod, e >> 1, mod)) * ((e & 1) ? a : 1) % mod));
}
template<class T, class U>
T nmod(T &x, U mod)
{
if (x >= mod) x -= mod;
}
template<class T>
T gcd(T a, T b)
{
return (b ? gcd(b, a % b) : a);
}
template<class T>
T randomize(T mod)
{
return (uniform_int_distribution<T>(0, mod - 1))(rng);
}
#define y0 ___y0
#define y1 ___y1
#define MP make_pair
#define MT make_tuple
#define PB push_back
#define PF push_front
#define fi first
#define se second
#define debug(x) cerr << #x << " = " << x << endl;
#define sz(x) ((int) (x.size()))
const long double PI = 4.0 * atan(1.0);
const long double EPS = 1e-9;
#define MAGIC 347
#define SINF 10007
#define CO 1000007
#define INF 1000000007
#define BIG 1000000931
#define LARGE 1696969696967ll
#define GIANT 2564008813937411ll
#define LLINF 2696969696969696969ll
#define MAXN 30013
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<ld, ld> pdd;
int N, M;
pii arr[MAXN];
vector<pii> edge[MAXN];
vector<int> vals[MAXN];
int S, T;
int dist[MAXN];
priority_queue<pii, vector<pii>, greater<pii> > pq;
int32_t main()
{
ios_base::sync_with_stdio(0); cin.tie(0);
// cout << fixed << setprecision(10);
// cerr << fixed << setprecision(10);
// freopen ("file.in", "r", stdin);
// freopen ("file.out", "w", stdout);
cin >> N >> M;
for (int i = 0; i < M; i++)
{
//loc, mod
cin >> arr[i].fi >> arr[i].se;
vals[arr[i].fi].PB(arr[i].se);
}
S = arr[0].fi; T = arr[1].fi;
for (int i = 0; i < N; i++)
{
sort(vals[i].begin(), vals[i].end());
vals[i].erase(unique(vals[i].begin(), vals[i].end()), vals[i].end());
}
for (int i = 0; i < N; i++)
{
for (int v : vals[i])
{
// cerr << i << ' ' << v << endl;
for (int j = i + v; j < N; j += v)
{
// cerr << "addedge " << i << ' ' << j << endl;
edge[i].PB({(j - i) / v, j});
if (binary_search(vals[j].begin(), vals[j].end(), v))
{
break;
}
}
for (int j = i - v; j >= 0; j -= v)
{
edge[i].PB({(i - j) / v, j});
if (binary_search(vals[j].begin(), vals[j].end(), v))
{
break;
}
}
}
}
for (int i = 0; i < N; i++)
{
dist[i] = INF;
}
dist[S] = 0;
pq.push({0, S});
while(!pq.empty())
{
int d = pq.top().fi, u = pq.top().se;
pq.pop();
if (u == T)
{
cout << d << '\n';
return 0;
}
// cerr << u << ' ' << dist[u] << ' ' << d << endl;
if (dist[u] != d)
{
continue;
}
// cerr << d << ' ' << u << endl;
for (pii p : edge[u])
{
int nd = d + p.fi, v = p.se;
if (dist[v] > nd)
{
dist[v] = nd;
pq.push({nd, v});
}
}
}
cout << (dist[T] == INF ? -1 : dist[T]) << '\n';
// cerr << "time elapsed = " << (clock() / (CLOCKS_PER_SEC / 1000)) << " ms" << endl;
return 0;
}
