#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define pf push_front
#define iter(v, i) for (__typeof__((v).begin()) i = (v).begin(); i != (v).end(); i++)
#define fast_io_without_cstdio ios_base::sync_with_stdio(false), cin.tie(NULL)
#define all(v) (v).begin(), (v).end()
#define rep(i, s, e) for (int i = s; i < e; i++)
// START for segment tree
#define params int p, int L, int R
#define housekeep int mid = (L + R) >> 1, left = p << 1, right = left | 1
// END
#ifdef __linux__
#define gc getchar_unlocked
#define pc putchar_unlocked
#else
#define gc getchar
#define pc putchar
#endif
#if __cplusplus <= 199711L
template<class BidirIt>
BidirIt prev(BidirIt it, typename iterator_traits<BidirIt>::difference_type n = 1) {
advance(it, -n);
return it;
}
template<class ForwardIt>
ForwardIt next(ForwardIt it, typename iterator_traits<ForwardIt>::difference_type n = 1) {
advance(it, n);
return it;
}
#endif
typedef long long ll;
typedef pair<int, int> ii;
typedef vector<int> vi;
typedef vector<ii> vii;
typedef long double ldouble;
const double EPS = 1e-9;
const double PI = 3.141592653589793238462;
template<typename T>
inline T sq(T a) { return a * a; }
//#ifdef LOCAL_MACHINE
//#endif
const int SIZE = 175;
const int MAXN = 3e4 + 5;
const int INF = 1e9;
bool dist[MAXN][SIZE];
deque<pair<int, ii> > deq;
bool visit[MAXN];
vi vec[MAXN];
int main() {
//freopen("", "r", stdin);
//freopen("", "w", stdout);
int n, m, st = -1, en = -1;
scanf("%d %d", &n, &m);
for (int i = 0; i < m; i++) {
int b, p;
scanf("%d %d", &b, &p);
vec[b].pb(p);
if (i == 0) {
st = b;
if (p < SIZE) {
dist[st][p] = true;
} else {
deq.pb(mp(0, mp(st, -p)));
}
deq.pb(mp(0, mp(st, p)));
}
if (i == 1) en = b;
}
//memset(dist, false, sizeof dist);
/*
for (int i = 0; i <= n; i++)
for (int j = 0; j < SIZE; j++)
dist[i][j] = INF;
*/
int ans = -1;
while (!deq.empty()) {
int w = deq.front().fi;
int u = deq.front().se.fi;
int x = deq.front().se.se;
deq.pop_front();
if (u == en) {
ans = w;
break;
}
if (x < 0 || x >= SIZE) {
if (0 <= u + x && u + x < n) {
deq.pb(mp(w + 1, mp(u + x, x)));
}
} else {
if (u - x >= 0 && !dist[u - x][x]) {
dist[u - x][x] = true;
deq.pb(mp(w + 1, mp(u - x, x)));
}
if (u + x < n && !dist[u + x][x]) {
dist[u + x][x] = true;
deq.pb(mp(w + 1, mp(u + x, x)));
}
}
if (visit[u]) continue;
visit[u] = true;
for (auto it : vec[u]) {
if (it < SIZE) {
if (!dist[u][it]) {
dist[u][it] = true;
deq.pf(mp(w, mp(u, it)));
}
} else {
deq.pf(mp(w, mp(u, it)));
deq.pf(mp(w, mp(u, -it)));
}
}
}
printf("%d\n", ans);
return 0;
}
