#include <bits/stdc++.h>
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
using namespace std;
typedef long long ll;
const int INF = 1e9;
const int B = 300;
const int MAXN = 3e4;
vector<pair<int, int> > d;
int dist[MAXN][B];
vector<array<int, 3> > g[MAXN][B];
int main()
{
//ifstream cin("input.txt");
//ofstream cout("output.txt");
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int n, m;
cin >> n >> m;
d.resize(m);
vector<int> big;
for (int i = 0; i < m; i++)
{
cin >> d[i].first >> d[i].second;
if (d[i].second >= B)
{
big.push_back(i);
}
}
for (int i = 0; i < n; i++)
for (int j = 0; j < B; j++)
dist[i][j] = INF;
for (int i = 0; i < m; i++)
{
if (d[i].second >= B) continue;
g[d[i].first][0].push_back({d[i].first, d[i].second, 0});
}
for (int i = 0; i < n; i++)
{
for (int j = 0; j < B; j++)
{
if (i - j >= 0)
{
g[i][j].push_back({i - j, 0, 1});
g[i][j].push_back({i - j, j, 1});
}
if (i + j < n)
{
g[i][j].push_back({i + j, 0, 1});
g[i][j].push_back({i + j, j, 1});
}
}
}
for (auto& i : big)
{
for (int j = d[i].first, k = 0; j < n; j += d[i].second, k++)
{
g[d[i].first][0].push_back({j, 0, k});
}
for (int j = d[i].first, k = 0; j >= 0; j -= d[i].second, k++)
{
g[d[i].first][0].push_back({j, 0, k});
}
}
set<array<int, 3> > st;
st.insert({0, d[0].first, 0});
dist[d[0].first][0] = 0;
while (!st.empty())
{
auto [dst, pos, pw] = *st.begin();
st.erase(st.begin());
for (auto& [psu, pwu, w] : g[pos][pw])
{
if (dist[psu][pwu] > dst + w)
{
st.erase({dist[psu][pwu], psu, pwu});
dist[psu][pwu] = dst + w;
st.insert({dist[psu][pwu], psu, pwu});
}
}
}
int mn = INF;
for (int i = 0; i < B; i++)
mn = min(mn, dist[d[1].first][i]);
if (mn == INF)
cout << -1;
else
cout << mn;
return 0;
}
/*
small
(i, j) -1> (i - j, 0), (i + j, 0)
(i, 0) -0> (i, j)
(i, j) -1> (i - j, j), (i + j, j)
large
(i, 0) -1> (i + j, 0)
(i, 0) -1> (i - j, 0)
*/
