/**
* code generated by JHelper
* More info: https://github.com/AlexeyDmitriev/JHelper
* @author win11905
*/
#include <bits/stdc++.h>
#define all(x) (x).begin(), (x).end()
#define vi vector<int>
#define iii tuple<int, int, int>
#define long long long
#define pii pair<int, int>
#define x first
#define y second
using namespace std;
const long MOD = 1e9+7, LINF = 1e18 + 1e16;
const int INF = 1e9+1;
const double EPS = 1e-10;
const int dx[4] = {-1, 0, 1, 0}, dy[4] = {0, 1, 0, -1};
const int N = 3e4+5;
const int SZ = 175;
class skyscraper {
private:
int n, m, s, t;
set<int> g[N];
int pre[SZ][N];
int d[N];
bool check[N];
public:
void solve(istream& cin, ostream& cout) {
cin >> n >> m;
for(int i = 0, a, b; i < m; ++i) {
cin >> a >> b;
g[a].emplace(b);
if(i == 0) s = a;
if(i == 1) t = a;
}
fill_n(d, N, INF);
fill_n(pre[0], SZ*N, INF);
priority_queue<pii, vector<pii>, greater<pii>> Q;
Q.emplace(d[s] = 0, s);
while(!Q.empty()) {
int u = Q.top().y, x = Q.top().x; Q.pop();
if(check[u]) continue; check[u] = true;
if(u == t) {
cout << x << endl;
return;
}
for(int p : g[u]) {
if(p < SZ) pre[p][u] = x;
for(int i = 1, v = u-p; v >= 0; v -= p, ++i) {
if(p < SZ) {
if(pre[p][v] <= x+i) break;
pre[p][v] = x+i;
}
if(d[v] > x+i) Q.emplace(d[v] = x+i, v);
}
for(int i = 1, v = u+p; v < n; v += p, ++i) {
if(p < SZ) {
if(pre[p][v] <= x+i) break;
pre[p][v] = x+1;
}
if(d[v] > x+i) Q.emplace(d[v] = x+i, v);
}
}
}
cout << -1 << endl;
}
};
class Solver {
public:
void solve(std::istream& in, std::ostream& out) {
skyscraper *obj = new skyscraper();
obj->solve(in, out);
}
};
int32_t main() {
ios::sync_with_stdio(false);
cin.tie(0);
Solver solver;
std::istream& in(std::cin);
std::ostream& out(std::cout);
solver.solve(in, out);
return 0;
}
