#include <bits/stdc++.h>
#pragma GCC optimize("O3")
#define FOR(i, x, y) for (ll i = x; i < y; i++)
typedef long long ll;
using namespace std;
struct Doge {
ll jumps, pos, size, direction;
};
vector<pair<ll, ll>> sky[300001];
bool visited[300001];
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
fill(visited, visited + 300001, false);
ll n, m;
cin >> n >> m;
ll size = 0, pos = 0;
FOR(i, 0, m) {
ll a, b;
cin >> a >> b;
sky[a].push_back({i, b});
if (i == 0) size = b, pos = a;
}
queue<Doge> q;
q.push({0, pos, size, 1});
q.push({0, pos, size, 0});
while (!q.empty()) {
Doge curr = q.front();
q.pop();
bool push = true;
// cout << curr.jumps << ' ' << curr.pos << ' ' << curr.size << ' '
// << curr.direction << '\n';
if (curr.pos > -1 && curr.pos < n) {
if (!visited[curr.pos]) {
visited[curr.pos] = true;
for (auto& i : sky[curr.pos]) {
if (i.first == 1) return cout << curr.jumps << '\n', 0;
if (i.second != curr.size) {
q.push(
{curr.jumps + 1, curr.pos + i.second, i.second, 1});
q.push(
{curr.jumps + 1, curr.pos - i.second, i.second, 0});
}
}
} else {
for (auto& i : sky[curr.pos]) {
if (i.second == curr.size) push = false;
}
}
}
if (push && curr.direction == 1 && curr.pos + curr.size < n)
q.push({curr.jumps + 1, curr.pos + curr.size, curr.size, 1});
else if (push && curr.direction == 0 && curr.pos - curr.size > -1)
q.push({curr.jumps + 1, curr.pos - curr.size, curr.size, 0});
}
cout << "-1\n";
return 0;
}
