#include <iostream>
#include <vector>
#include <queue>
#include <tuple>
using namespace std;
int n, m;
vector<int> b, p;
vector<vector<int>> v;
int solve() {
vector<int> dist(m, -1);
priority_queue<tuple<int, int, int>> q;
dist[b[0]] = 0;
q.push({0, b[0], 0});
while (not q.empty() and dist[1] == -1) {
auto [d, i, j] = q.top();
q.pop();
while (not v[i].empty()) {
dist[v[i].back()] = -d;
q.push(make_tuple(d, i, v[i].back()));
v[i].pop_back();
}
if (i >= b[j] and i + p[j] < n) q.push(make_tuple(d - 1, i + p[j], j));
if (i <= b[j] and i - p[j] >= 0) q.push(make_tuple(d - 1, i - p[j], j));
}
return dist[1];
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin >> n >> m;
v.resize(n), b.resize(m), p.resize(m);
cin >> b[0] >> p[0];
for (int i = 1; i < m; i++) {
cin >> b[i] >> p[i];
v[b[i]].push_back(i);
}
cout << solve() << '\n';
return 0;
}
