#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9;
const int BLK = 200;
int n, m;
vector<int> b, p;
vector<vector<int>> v;
void addDoge(int i, int pos, int d, vector<int> &dist, priority_queue<pair<int, int>> &q, bool dir) {
int c = (dir ? 1 : -1);
for (int pos_nxt = pos + c * p[i], x = 1; (dir ? pos_nxt < n : pos_nxt >= 0); pos_nxt += c * p[i], x++) {
if (-d + x < dist[pos_nxt]) {
dist[pos_nxt] = -d + x;
q.push({d - x, pos_nxt});
}
}
}
int solve() {
vector<int> dist(n, INF);
priority_queue<pair<int, int>> q;
dist[b[0]] = 0;
q.push({0, b[0]});
while (not q.empty()) {
auto [d, pos] = q.top();
q.pop();
if (dist[pos] != -d) continue;
while (not v[pos].empty()) {
addDoge(v[pos].back(), pos, d, dist, q, 0);
addDoge(v[pos].back(), pos, d, dist, q, 1);
v[pos].pop_back();
}
// if (power and pos + power < n and -d + 1 < dist[pos + power][power]) {
// dist[pos + power][power] = -d + 1;
// q.push(make_tuple(d - 1, pos + power, power));
// }
// if (power and pos - power >= 0 and -d + 1 < dist[pos - power][power]) {
// dist[pos - power][power] = -d + 1;
// q.push(make_tuple(d - 1, pos - power, power));
// }
}
int ret = INF;
for (int i = 0; i < BLK; i++) {
ret = min(ret, dist[b[1]]);
}
return ret == INF ? -1 : ret;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin >> n >> m;
v.resize(n), b.resize(m), p.resize(m);
for (int i = 0; i < m; i++) {
cin >> b[i] >> p[i];
v[b[i]].push_back(i);
}
cout << solve() << '\n';
return 0;
}
