#include <cmath>
#include <functional>
#include <fstream>
#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <set>
#include <map>
#include <list>
#include <time.h>
#include <math.h>
#include <random>
#include <deque>
#include <queue>
#include <unordered_map>
#include <unordered_set>
#include <iomanip>
#include <cassert>
#include <bitset>
#include <sstream>
#include <chrono>
#include <cstring>
#include <numeric>
using namespace std;
const int INF = (int)1e9 + 7;
struct T
{
int pos;
int delta;
};
bool operator < (T a, T b)
{
return make_pair(a.pos, a.delta) < make_pair(b.pos, b.delta);
}
signed main()
{
#ifdef ONPC
FILE* stream;
freopen_s(&stream, "input.txt", "r", stdin);
#else
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#endif
int dim, cntofs;
cin >> dim >> cntofs;
vector<T> ofs(cntofs);
vector<vector<int>> cine(dim);
for (int i = 0; i < cntofs; i++)
{
cin >> ofs[i].pos >> ofs[i].delta;
assert(0 <= ofs[i].pos && ofs[i].pos < dim);
}
int Init = ofs[0].pos;
int Query = ofs[1].pos;
sort(ofs.begin(), ofs.end());
vector<vector<int>> deltas(dim);
{
assert(!ofs.empty());
vector<T> newofs = { ofs[0] };
for (int i = 1; i < cntofs; i++)
{
if (make_pair(ofs[i].pos, ofs[i].delta) != make_pair(ofs[i - 1].pos, ofs[i - 1].delta))
{
newofs.push_back(ofs[i]);
}
}
ofs = newofs;
cntofs = (int)ofs.size();
for (auto& of : ofs)
{
deltas[of.pos].push_back(of.delta);
}
for (auto& delta : deltas)
{
sort(delta.begin(), delta.end());
}
}
auto has = [&](int pos, int delta)
{
int low = 0, high = (int)deltas[pos].size() - 1;
while (low <= high)
{
int mid = (low + high) / 2;
if (deltas[pos][mid] == delta)
{
return 1;
}
if (deltas[pos][mid] < delta)
{
low = mid + 1;
}
else
{
high = mid - 1;
}
}
return 0;
};
assert((int)ofs.size() == cntofs);
for (int i = 0; i < cntofs; i++)
{
assert(0 <= ofs[i].pos && ofs[i].pos < dim);
cine[ofs[i].pos].push_back(i);
}
vector<int> dp(dim, INF);
priority_queue<pair<int, int>> pq;
auto relax = [&](int i, int val)
{
if (val < dp[i])
{
dp[i] = val;
pq.push({ -dp[i], i });
}
};
set<pair<int, int>> s_delta_rest;
// {delta, rest}
for (auto& it : ofs)
{
s_delta_rest.insert({ it.delta, it.pos % it.delta });
}
vector<pair<int, int>> v_delta_rest;
for (auto& it : s_delta_rest)
{
v_delta_rest.push_back(it);
}
int states = (int)v_delta_rest.size();
vector<vector<int>> cool(states);
for (int it = 0; it < states; it++)
{
int i = v_delta_rest[it].first, j = v_delta_rest[it].second;
for (int p = j; p < dim; p += i)
{
if (!cine[p].empty())
{
cool[it].push_back(p);
}
}
}
auto fnd = [&](pair<int, int> it)
{
assert(0 <= it.second && it.second < it.first);
int l = 0, r = (int)v_delta_rest.size() - 1;
while (l <= r)
{
int m = (l + r) / 2;
if (v_delta_rest[m] == it)
{
return m;
}
if (v_delta_rest[m] < it)
{
l = m + 1;
}
else
{
r = m - 1;
}
}
assert(0);
};
relax(Init, 0);
while (!pq.empty())
{
auto it = pq.top();
pq.pop();
int loc = it.second;
if (-it.first != dp[loc])
{
continue;
}
for (auto& i : cine[loc])
{
int what = fnd({ ofs[i].delta, loc % ofs[i].delta });
int pz = lower_bound(cool[what].begin(), cool[what].end(), loc) - cool[what].begin();
assert(0 <= pz && pz < (int)cool[what].size());
assert(cool[what][pz] == loc);
for (int j = pz - 1; j >= 0; j--)
{
int loc2 = cool[what][j];
int posdif = abs(ofs[i].pos - loc2);
relax(loc2, dp[loc] + posdif / ofs[i].delta);
if (has(cool[what][j], ofs[i].delta)) break;
}
for (int j = pz + 1; j < (int)cool[what].size(); j++)
{
int loc2 = cool[what][j];
int posdif = abs(ofs[i].pos - loc2);
relax(loc2, dp[loc] + posdif / ofs[i].delta);
if (has(cool[what][j], ofs[i].delta)) break;
}
}
}
int sol = (dp[Query] == INF) ? (-1) : (dp[Query]);
cout << sol << "\n";
return 0;
}
