#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for(int i = 1; i <= n; ++i)
#define forn(i, l, r) for(int i = l; i <= r; ++i)
#define ford(i, r, l) for(int i = r; i >= l; --i)
#define FOR(i, n) for(int i = 0; i < n; ++i)
#define FORD(i, n) for(int i = n - 1; i >= 0; --i)
#define fi first
#define se second
#define pii pair<int, int>
#define pll pair<ll, ll>
#define pb push_back
#define endl "\n"
#define task "skycrapers"
#define sz(a) int(a.size())
#define C(x, y) make_pair(x, y)
#define all(a) (a).begin(), (a).end()
#define bit(i, mask) (mask >> i & 1)
template<typename T> bool maximize(T &res, const T &val) { if (res < val){ res = val; return true; }; return false; }
template<typename T> bool minimize(T &res, const T &val) { if (res > val){ res = val; return true; }; return false; }
inline int readInt() {char c;while(c=getchar(),c!='-'&&(c<'0'||c>'9'));bool sign=(c=='-');if(sign)c=getchar();int n=c-'0';while(c=getchar(),c>='0'&&c<='9')n=10*n+c-'0';return(!sign)?n:-n;}
inline ll readLong() {char c;while(c=getchar(),c!='-'&&(c<'0'||c>'9'));bool sign=(c=='-');if(sign)c=getchar();ll n=c-'0';while(c=getchar(),c>='0'&&c<='9')n=10*n+c-'0';return(!sign)?n:-n;}
inline string readString() {char c;while(c=getchar(),c==' '||c=='\n'||c=='\t');string s({c});while(c=getchar(),c!=EOF&&c!=' '&&c!='\n'&&c!='\t')s+=c;return s;}
const int N = 1e5 + 10;
const int M = 1e3 + 3;
const int N1 = 2e3 + 10;
const int K = 1e2 + 1;
const int MOD = 1e9 + 7;
const int INF = 1e9 + 3;
const ll LINF = 1e17 + 2;
const int block_size = 500;
const int LOG = 40;
const int offset = N;
const int LIM = 11 ;
const int AL = 26;
int n, m, s, t;
set<int> doges[N];
int d[N];
void solve()
{
cin >> n >> m;
FOR(i, m)
{
int x, y;
cin >> x >> y;
doges[x].insert(y);
if(i == 0) s = x;
else if(i == 1) t = x;
}
priority_queue<pii, vector<pii>, greater<pii>> pq;
FOR(i, n) d[i] = INF;
d[s] = 0; pq.push({0, s});
while(!pq.empty())
{
pii tmp = pq.top();
pq.pop();
int u = tmp.se, dist = tmp.fi;
if(d[u] != dist) continue;
if(u == t) return cout << dist, void();
for(int p : doges[u])
{
for(int j = 1; u + p * j < n; ++j)
{
int v = u + p * j;
if(minimize(d[v], d[u] + j))
pq.push({d[v], v});
if(doges[v].find(p) != doges[v].end()) break;
}
for(int j = 1; u - p * j >= 0; ++j)
{
int v = u - p * j;
if(minimize(d[v], d[u] + j))
pq.push({d[v], v});
if(doges[v].find(p) != doges[v].end()) break;
}
}
}
cout << -1;
}
signed main()
{
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int TC = 1;
if(fopen(task".inp", "r"))
{
freopen(task".inp", "r", stdin);
freopen(task".out", "w", stdout);
}
while(TC--)
{
solve();
cout << endl;
}
return 0;
}
