#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 = 350;
const int MAXN = 3e4 + 5;
vector<pair<int, int> > d;
int dist[MAXN][B];
vector<short> sm[MAXN];
vector<short> bg[MAXN];
signed 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);
bg[d[i].first].push_back(d[i].second);
}
else
{
sm[d[i].first].push_back(d[i].second);
}
}
for (int i = 0; i < n; i++)
for (int j = 0; j < B; j++)
dist[i][j] = INF;
priority_queue<array<int, 3> > st;
st.push({0, d[0].first, 0});
dist[d[0].first][0] = 0;
while (!st.empty())
{
auto [dst, pos, pw] = st.top();
dst = -dst;
st.pop();
if (dst > dist[pos][pw]) continue;
if (dst != dist[pos][pw]) continue;
if (pw > 0)
{
vector<array<short, 3> > to;
if (pos - pw >= 0)
{
if (dist[pos - pw][pw] > dst + 1)
{
//st.erase({dist[pos - pw][pw], pos - pw, pw});
dist[pos - pw][pw] = dst + 1;
st.push({-dist[pos - pw][pw], pos - pw, pw});
}
if (dist[pos - pw][0] > dst + 1)
{
//st.erase({dist[pos - pw][0], pos - pw, 0});
dist[pos - pw][0] = dst + 1;
st.push({-dist[pos - pw][0], pos - pw, 0});
}
}
if (pos + pw < n)
{
if (dist[pos + pw][pw] > dst + 1)
{
//st.erase({dist[pos + pw][pw], pos + pw, pw});
dist[pos + pw][pw] = dst + 1;
st.push({-dist[pos + pw][pw], pos + pw, pw});
}
if (dist[pos + pw][0] > dst + 1)
{
//st.erase({dist[pos + pw][0], pos + pw, 0});
dist[pos + pw][0] = dst + 1;
st.push({-dist[pos + pw][0], pos + pw, 0});
}
}
continue;
}
for (auto& mv : bg[pos])
{
for (int j = pos, k = 0; j < n; j += mv, k++)
{
if (dist[j][0] > dst + k)
{
dist[j][0] = dst + k;
st.push({-dist[j][0], j, 0});
}
}
for (int j = pos, k = 0; j >= 0; j -= mv, k++)
{
if (dist[j][0] > dst + k)
{
dist[j][0] = dst + k;
st.push({-dist[j][0], j, 0});
}
}
}
for (auto& mv : sm[pos])
{
if (dist[pos][mv] > dst + 0)
{
dist[pos][mv] = dst + 0;
st.push({-dist[pos][mv], pos, mv});
}
}
}
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)
*/
