#include <bits/stdc++.h>
#define SZ(x) ((int)x.size())
#define fi first
#define se second
#define pb push_back
#define FOR(i, a, b) for (int i = a, _b = b; i <= _b; ++i)
#define FORD(i, a, b) for (int i = a, _b = b; i >= _b; --i)
#define FORLL(i, a, b) for (ll i = a, _b = b; i <= _b; ++i)
#define FORDLL(i, a, b) for (ll i = a, _b = b; i >= _b; --i)
#define all(x) x.begin(), x.end()
#define uni(x) sort(all(x)), x.erase(unique(all(x)), x.end())
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define dbg(...) debug(#__VA_ARGS__, __VA_ARGS__)
template<typename T>
void __print_one(const char *&s, const T &x)
{
while (*s == ' ') ++s;
const char *p = s;
int bal = 0;
while (*s)
{
if (*s == '(') ++bal;
else if (*s == ')') --bal;
else if (*s == ',' && bal == 0) break;
++s;
}
cerr.write(p, s - p) << " = " << x;
if (*s == ',')
{
++s;
cerr << " , ";
}
}
template<typename... Args>
void debug(const char *s, Args... args)
{
cerr << "[ ";
int dummy[] = { 0 , ( __print_one(s, args) , 0 )... };
(void)dummy;
cerr << " ]\n\n";
}
template<class X>
bool maximize(X &a, X b)
{
if (a < b)
{
a = b;
return true;
}
return false;
}
template<class X>
bool minimize(X &a, X b)
{
if (a > b)
{
a = b;
return true;
}
return false;
}
// --------------------------------------------------------------------------------------------
constexpr int maxn = 3e4 + 3;
int n, m, P[maxn], B[maxn], res = 1e9;
vector<int> Dogs[maxn];
bitset<maxn> vis;
bitset<maxn> vis_dist[maxn];
// --------------------------------------------------------------------------------------------
struct Obj
{
int u, d, w;
};
queue<Obj> Q;
bool valid(int x)
{
return x >= 0 && x <= n - 1;
}
bool connect(int u, int d, int w, int v)
{
if (!valid(v)) return false;
if (vis_dist[v][d]) return false;
// dbg(u, d);
Q.push({v, d, w + 1});
vis_dist[v][d] = true;
if (vis[v] == false)
{
for (int x : Dogs[v])
{
if (vis_dist[v][P[x]] == false)
{
Q.push({v, P[x], w + 1});
vis_dist[v][P[x]] = true;
}
}
vis[v] = true;
}
if (v == B[1])
{
res = w + 1;
return true;
}
return false;
}
void solve()
{
cin >> n >> m;
FOR(i, 0, m - 1)
{
cin >> B[i] >> P[i];
Dogs[B[i]].push_back(i);
}
if (B[0] == B[1])
{
res = 0;
goto LAST;
}
vis[B[0]] = true;
for (int x : Dogs[B[0]])
{
if (vis_dist[B[0]][P[x]] == false)
{
Q.push({B[0], P[x], 0});
vis_dist[B[0]][P[x]] = true;
}
}
while (Q.size())
{
int u = Q.front().u, d = Q.front().d, w = Q.front().w; Q.pop();
if (connect(u, d, w, u + d))
break;
if (connect(u, d, w, u - d))
break;
}
LAST:
;
res = (res == int(1e9) ? -1 : res);
cout << res;
dbg(res);
}
signed main()
{
ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define TASK "TEST"
if (fopen(TASK".INP", "r"))
{
freopen(TASK".INP", "r", stdin);
freopen(TASK".OUT", "w", stdout);
}
solve();
return 0;
}
