//In The Name Of Allah
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
//typedef sh short int
#define endl '\n'
#define pb push_back
#define F first
#define S second
#define all(x) x.begin(), x.end()
#pragma GCC optimize("Ofast")
const int N = 6e4 + 5, M = 6e4 + 5, inf = 1e9 + 5;
int n, m, b[M], p[M], dis[M], mm = 2;
vector<int> adj[M];
vector<int> pos[N];
set<pair<int, int> > bp;
int main()
{
ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
cin >> n >> m;
for(int i = 0; i < m; i++)
{
cin >> b[i] >> p[i];
if(i > 1)
bp.insert({b[i], p[i]});
}
for(auto x : bp)
{
bool is = 1;
if(is)
{
b[mm] = x.F;
p[mm] = x.S;
mm++;
}
}
for(int i = 0; i < mm; i++)
pos[b[i]].pb(i);
for(int i = 0; i < mm; i++)
{
int ptr = b[i];
while(ptr < n)
{
for(auto x : pos[ptr])
adj[i].pb(x);
ptr += p[i];
}
ptr = b[i] - p[i];
while(ptr >= 0)
{
for(auto x : pos[ptr])
adj[i].pb(x);
ptr -= p[i];
}
}
for(int i = 1; i < mm + 5; i++)
dis[i] = inf;
set<pair<int, int> > st;
st.insert({0, 0});
while(st.size())
{
auto x = *st.begin();
//cout << x.S << endl;
st.erase(x);
for(auto v : adj[x.S])
{
int dist = abs(b[v] - b[x.S]) / p[x.S];
if(x.F + dist < dis[v])
{
dis[v] = x.F + dist;
st.insert({dis[v], v});
}
}
}
if(dis[1] == inf)
cout << -1 << endl;
else
cout << dis[1] << endl;
//cout << dis[1] << endl;
}
