#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
/*typedef __uint128_t L;
struct FastMod {
ull b, m;
FastMod(ull b) : b(b), m(ull((L(1) << 64) / b)) {}
ull reduce(ull a) {
ull q = (ull)((L(m) * a) >> 64);
ull r = a - q * b; // can be proven that 0 <= r < 2*b
return r >= b ? r - b : r;
}
};
FastMod FM(2);*/
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
cerr << " " << H;
debug_out(T...);
}
#define debug(...) cerr << "(" << #__VA_ARGS__ << "):", debug_out(__VA_ARGS__)
#define all(x) x.begin(), x.end()
#define MP(x, y) make_pair(x, y)
#define F first
#define S second
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const int maxn = 3e4 + 10;
int n, m, a[maxn], b[maxn], h[maxn];
map<pii,bool> mp;
vector<int> val[maxn];
void dijkstra(int st){
memset(h, 63, sizeof h);
h[st] = 0;
priority_queue<pii, vector<pii>, greater<pii>> q;
q.push({0, st});
while(!q.empty()){
int v = q.top().S, w = q.top().F;
q.pop();
if (w > h[v]) continue;
for (auto x: val[v]){
int tmp = v % x;
if (mp[{x, tmp}]) continue;
mp[{x, tmp}] = true;
for (int i = 1; v + i * x < n; i++){
int u = v + i * x;
if (h[u] > h[v] + i){
h[u] = h[v] + i;
q.push({h[u], u});
}
}
for (int i = 1; v - i * x >= 0; i++){
int u = v - i * x;
if (h[u] > h[v] + i){
h[u] = h[v] + i;
q.push({h[u], u});
}
}
}
}
}
int main(){
ios_base::sync_with_stdio(false); cin.tie(0);
cin >> n >> m;
for (int i = 1; i <= m; i++){
cin >> a[i] >> b[i];
val[a[i]].push_back(b[i]);
}
dijkstra(a[1]);
cout << (h[a[2]] > n? -1: h[a[2]]) << '\n';
return 0;
}
