//#define _GLIBCXX_DEBUG
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include <bits/stdc++.h>
using namespace std;
//#include <ext/pb_ds/assoc_container.hpp>
//
//using namespace __gnu_pbds;
//
//template<typename T>
//using ordered_set = tree<T, null_type, less < T>, rb_tree_tag, tree_order_statistics_node_update>;
//template<typename T>
//using normal_queue = priority_queue<T, vector<T>, greater<>>;
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
#define trace(x) cout << #x << ": " << (x) << endl;
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
#define uniq(x) x.resize(unique(all(x)) - begin(x))
#define sz(s) ((int) size(s))
#define pii pair<int, int>
#define mp(x, y) make_pair(x, y)
#define int128 __int128
#define pb push_back
#define popb pop_back
#define eb emplace_back
#define fi first
#define se second
#define itn int
typedef long long ll;
typedef pair<ll, ll> pll;
typedef long double ld;
typedef double db;
typedef unsigned int uint;
template<typename T>
bool ckmn(T &x, T y) {
if (x > y) {
x = y;
return true;
}
return false;
}
template<typename T>
bool ckmx(T &x, T y) {
if (x < y) {
x = y;
return true;
}
return false;
}
int bit(int x, int b) {
return (x >> b) & 1;
}
int rand(int l, int r) { return (int) ((ll) rnd() % (r - l + 1)) + l; }
const ll infL = 3e18;
const int infI = 1000000000 + 7;
const int infM = 0x3f3f3f3f; //a little bigger than 1e9
const ll infML = 0x3f3f3f3f3f3f3f3fLL; //4.5e18
const int N = 30002;
const int mod = 998244353;
const ld eps = 1e-9;
vector<pair<int, int>> g[N * 2];
vector<int> here[N];
int b[N], p[N], dist[N * 2];
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n, m;
cin >> n >> m;
for (int i = 0; i < m; ++i) {
cin >> b[i] >> p[i];
assert(b[i] < n);
here[b[i]].push_back(i);
}
set<pair<int, int>> st;
fill(dist, dist + N * 2, infI);
st.insert(mp(0, 0));
dist[0] = 0;
while (!st.empty()) {
auto [W, v] = *st.begin();
st.erase(st.begin());
if (v < m) {
{
int cost = 0;
int to = b[v] + m;
while (to < n + m) {
if (dist[to] > dist[v] + cost) {
st.erase(mp(dist[to], to));
dist[to] = dist[v] + cost;
st.insert(mp(dist[to], to));
}
cost += 1;
to += p[v];
}
}
{
int cost = 0;
int to = b[v] + m;
while (to > m - 1) {
if (dist[to] > dist[v] + cost) {
st.erase(mp(dist[to], to));
dist[to] = dist[v] + cost;
st.insert(mp(dist[to], to));
}
cost += 1;
to -= p[v];
}
}
} else {
for (int x : here[v - m]) {
if (dist[x] > dist[v]) {
st.erase(mp(dist[x], x));
dist[x] = dist[v];
st.insert(mp(dist[x], x));
}
}
}
}
cout << (dist[1] == infI ? -1 : dist[1]);
return 0;
}
