#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define FOR(i, a, b) for (int i = (a), _b = (b); i <= _b; i++)
#define FORD(i, b, a) for (int i = (b), _a = (a); i >= _a; i--)
#define pa pair<ll, ll>
#define fi first
#define se second
#define bit(mask, j) ((mask >> j) & 1)
#define t_test int t;cin >> t;while(t--)
const ll mod = 1e9 + 7;
const ll INF = 1e18;
inline void adm(ll &x){if(x>=mod)x%=mod;else if(x<0)x+=mod;}
//--------------------------------------------------------------------
struct state {
ll w, u, t;
bool operator < (const state &a) const {
return w > a.w;
}
};
priority_queue<state> q;
const ll BLOCK = 180;
const ll N = 3e4 + 10;
vector<ll> g[N];
ll n, m;
ll dp[N][BLOCK + 10];
void hbmt() {
cin >> n >> m;
ll doge1 = 0;
memset(dp, 0x3f, sizeof dp);
FOR(i, 1, m) {
ll b, p;
cin >> b >> p;
if(i == 1) {
dp[b][0] = 0;
q.push({0, b, 0});
}
if(i == 2) {
doge1 = b;
}
g[b].push_back(p);
}
while(!q.empty()) {
ll u = q.top().u, du = q.top().w, t = q.top().t;
q.pop();
if(du != dp[u][t]) continue;
if(u == doge1) {
cout << du;
exit(0);
}
if(t != 0 && u + t < n) {
if(dp[u + t][t] > dp[u][t] + 1) {
dp[u + t][t] = dp[u][t] + 1;
q.push({dp[u + t][t], u + t, t});
}
}
if(t != 0 && u - t >= 0) {
if(dp[u - t][t] > dp[u][t] + 1) {
dp[u - t][t] = dp[u][t] + 1;
q.push({dp[u - t][t], u - t, t});
}
}
for(auto p : g[u]) {
if(p > BLOCK) {
ll c = 0;
for(int v = u + p; v < n; v += p) {
c++;
if(dp[v][0] > dp[u][t] + c) {
dp[v][0] = dp[u][t] + c;
q.push({dp[v][0], v, 0});
}
}
c = 0;
for(int v = u - p; v >= 0; v -= p) {
c++;
if(dp[v][0] > dp[u][t] + c) {
dp[v][0] = dp[u][t] + c;
q.push({dp[v][0], v, 0});
}
}
}
else if(p != 0) {
if(u + p < n) {
if(dp[u + p][p] > dp[u][t] + 1) {
dp[u + p][p] = dp[u][t] + 1;
q.push({dp[u + p][p], u + p, p});
}
}
if(u - p >= 0) {
if(dp[u - p][p] > dp[u][t] + 1) {
dp[u - p][p] = dp[u][t] + 1;
q.push({dp[u - p][p], u - p, p});
}
}
}
}
}
cout << -1;
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);cout.tie(0);
if(fopen("hbmt.inp", "r")) {
freopen("hbmt.inp", "r", stdin);
freopen("hbmt.out", "w", stdout);
}
// t_test
hbmt();
return 0;
}
