// ~~ icebear love attttt ~~
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> ii;
typedef pair<int, ii> iii;
template<class T>
bool minimize(T &a, const T &b) {
if (a > b) return a = b, true;
return false;
}
template<class T>
bool maximize(T &a, const T &b) {
if (a < b) return a = b, true;
return false;
}
#define FOR(i,a,b) for(int i=(a); i<=(b); ++i)
#define FORR(i,a,b) for(int i=(a); i>=(b); --i)
#define REP(i, n) for(int i=0; i<(n); ++i)
#define RED(i, n) for(int i=(n)-1; i>=0; --i)
#define MASK(i) (1LL << (i))
#define BIT(S, i) (((S) >> (i)) & 1)
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define all(x) x.begin(), x.end()
#define task "icebearat"
#define index loveat
const int MOD = 1e9 + 7;
const int inf = 1e9 + 27092008;
const ll INF = 1e18 + 27092008;
const int N = 30000 + 5;
int n, m;
vector<int> G[N];
int dist[N], index[N], far[N];
map<int, bool> have[N];
void init(void) {
cin >> n >> m;
REP(i, m) {
cin >> index[i] >> far[i];
G[index[i]].pb(far[i]);
have[index[i]][far[i]] = true;
}
REP(i, n) {
sort(all(G[i]));
G[i].resize(unique(all(G[i])) - G[i].begin());
}
}
void process(void) {
priority_queue<ii, vector<ii>, greater<ii>> Q;
memset(dist, 0x3f, sizeof dist);
dist[index[0]] = 0;
Q.push(mp(0, index[0]));
while(!Q.empty()) {
int du, u; tie(du, u) = Q.top();
Q.pop();
if (du != dist[u]) continue;
if (u == index[1]) {
cout << du;
return;
}
for(int jump : G[u]) {
for(int w = 1; u + w * jump < n; w++) {
int v = u + w * jump;
if (!have[v].empty() && minimize(dist[v], du + w))
Q.push(mp(du + w, v));
if (have[v].find(jump) != have[v].end()) break;
}
for(int w = 1; u - w * jump >= 0; w++) {
int v = u - w * jump;
if (!have[v].empty() && minimize(dist[v], du + w))
Q.push(mp(du + w, v));
if (have[v].find(jump) != have[v].end()) break;
}
}
}
cout << -1;
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
if (fopen(task".inp", "r")) {
freopen(task".inp", "r", stdin);
freopen(task".out", "w", stdout);
}
int tc = 1;
// cin >> tc;
while(tc--) {
init();
process();
}
return 0;
}
