#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn = 30000 + 10;
const int inf = 1e9;
const int T = 120;
int b[maxn], p[maxn], dp[maxn * T];
int n, m;
vector<pair<int, int> > g[maxn * T];
int id(int v, int x){
return v * T + x;
}
void dijkstra(int v){
for (int i = 0; i < n * T; i++)
dp[i] = inf;
dp[v] = 0;
set<pair<int, int> > s;
for (int i = 0; i < n * T; i++)
s.insert({dp[i], i});
while (!s.empty()){
int v = (*s.begin()).second;
s.erase(s.begin());
for (auto edge : g[v]){
int u = edge.first, w = edge.second;
if (dp[u] > dp[v] + w){
s.erase({dp[u], u});
dp[u] = dp[v] + w;
s.insert({dp[u], u});
}
}
}
}
int main(){
ios_base::sync_with_stdio(false);
cin >> n >> m;
for (int i = 0; i < m; i++){
cin >> b[i] >> p[i];
if (p[i] < T)
g[id(b[i], 0)].push_back({id(b[i], p[i]), 0});
else{
for (int u = b[i] + p[i]; u < n; u += p[i])
g[id(b[i], 0)].push_back({id(u, 0), (u - b[i]) / p[i]});
for (int u = b[i] - p[i]; u >= 0; u -= p[i])
g[id(b[i], 0)].push_back({id(u, 0), (b[i] - u) / p[i]});
}
}
for (int i = 0; i < n; i++){
for (int j = 1; j < T; j++){
g[id(i, j)].push_back({id(i, 0), 0});
if (i + j < n)
g[id(i, j)].push_back({id(i + j, j), 1});
if (i - j >= 0)
g[id(i, j)].push_back({id(i - j, j), 1});
}
}
dijkstra(id(b[0], 0));
if (dp[id(b[1], 0)] == inf)
dp[id(b[1], 0)] = -1;
cout << dp[id(b[1], 0)] << '\n';
}
