#include <iostream>
#include <vector>
#include <algorithm>
#include <deque>
#include <cassert>
using namespace std;
const short INF = 32767;
int main()
{
cin.tie(0)->sync_with_stdio(0);
int n, m;
cin >> n >> m;
vector<pair<int, int>> doges(m);
for (auto &[b, p] : doges) cin >> b >> p;
const short D = 180;
vector<vector<short>> dp(n, vector<short>(D, INF));
vector<vector<short>> canGet(n), starts(n);
if (n <= 2000)
{
for (auto &[b, p] : doges)
{
starts[b].push_back(p);
if (p >= D) for (int i = b % p; i < n; i += p) canGet[i].push_back(p);
}
for (int i = 0; i < n; ++i)
{
sort(canGet[i].begin(), canGet[i].end());
canGet[i].erase(unique(canGet[i].begin(), canGet[i].end()), canGet[i].end());
sort(starts[i].begin(), starts[i].end());
starts[i].erase(unique(starts[i].begin(), starts[i].end()), starts[i].end());
for (int j = 0; j < canGet.size(); ++j) dp[i].push_back(INF);
}
auto getInd = [&canGet, &D](short v, short p) -> short
{
if (p < D) return p;
return (short)distance(canGet[v].begin(), lower_bound(canGet[v].begin(), canGet[v].end(), p)) + (short)D;
};
short ind = getInd(doges[0].first, doges[0].second);
dp[doges[0].first][ind] = 0;
deque<pair<short, short>> dq;
dq.emplace_back(doges[0].first, doges[0].second);
while (!dq.empty())
{
auto [i, p] = dq.front();
dq.pop_front();
short ind = getInd(i, p);
for (int j : starts[i])
{
int ind2 = getInd(i, j);
if (dp[i][ind2] > dp[i][ind])
{
dp[i][ind2] = dp[i][ind];
dq.emplace_front(i, j);
}
}
starts[i].clear();
if (i - p >= 0)
{
short ind2 = getInd(i - p, p);
if (dp[i - p][ind2] > dp[i][ind] + 1)
{
dp[i - p][ind2] = dp[i][ind] + 1;
dq.emplace_back(i - p, p);
}
}
if (i + p < n)
{
short ind2 = getInd(i + p, p);
if (dp[i + p][ind2] > dp[i][ind] + 1)
{
dp[i + p][ind2] = dp[i][ind] + 1;
dq.emplace_back(i + p, p);
}
}
}
ind = getInd(doges[1].first, doges[1].second);
if (dp[doges[1].first][ind] == INF) cout << -1;
else cout << dp[doges[1].first][ind];
}
return 0;
}
