#include <bits/stdc++.h>
using namespace std;
int a, b;
vector<int> z[1000005];
int blocksize;
pair<int, int> mark[30005];
struct node {
int val, pos, type;
bool operator>(const node &other) const {
return val > other.val;
}
};
map<pair<int, int>, int> cnt;
void dijkstra() {
priority_queue<node, vector<node>, greater<node>> q;
q.push({0, mark[0].first, mark[0].second});
cnt[{mark[0].first, mark[0].second}] = 0;
while (!q.empty()) {
node pos = q.top();
q.pop();
int val = pos.val;
int pos1 = pos.pos;
int type = pos.type;
if (cnt[{pos1, type}] < val) continue;
if (type == 0) {
for (auto p : z[pos1]) {
if (p + pos1 < a) {
if (!cnt.count({p + pos1, p}) || cnt[{p + pos1, p}] > val + 1) {
cnt[{p + pos1, p}] = val + 1;
q.push({cnt[{p + pos1, p}], p + pos1, p});
}
}
if (pos1 - p >= 0) {
if (!cnt.count({pos1 - p, p}) || cnt[{pos1 - p, p}] > val + 1) {
cnt[{pos1 - p, p}] = val + 1;
q.push({cnt[{pos1 - p, p}], pos1 - p, p});
}
}
}
} else {
if (!cnt.count({pos1, 0}) || cnt[{pos1, 0}] > val) {
cnt[{pos1, 0}] = val;
q.push({val, pos1, 0});
}
if (type > blocksize) {
int cnt1 = 1;
while (pos1 + cnt1 * type < a) {
int nxt = pos1 + cnt1 * type;
if (!cnt.count({nxt, 0}) || cnt[{nxt, 0}] > val + cnt1) {
cnt[{nxt, 0}] = val + cnt1;
q.push({cnt[{nxt, 0}], nxt, 0});
}
cnt1++;
}
cnt1 = 1;
while (pos1 - cnt1 * type >= 0) {
int nxt = pos1 - cnt1 * type;
if (!cnt.count({nxt, 0}) || cnt[{nxt, 0}] > val + cnt1) {
cnt[{nxt, 0}] = val + cnt1;
q.push({cnt[{nxt, 0}], nxt, 0});
}
cnt1++;
}
} else {
int nxt1 = pos1 + type;
int nxt2 = pos1 - type;
if (nxt1 < a && (!cnt.count({nxt1, type}) || cnt[{nxt1, type}] > val + 1)) {
cnt[{nxt1, type}] = val + 1;
q.push({cnt[{nxt1, type}], nxt1, type});
}
if (nxt2 >= 0 && (!cnt.count({nxt2, type}) || cnt[{nxt2, type}] > val + 1)) {
cnt[{nxt2, type}] = val + 1;
q.push({cnt[{nxt2, type}], nxt2, type});
}
}
}
}
}
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin >> a >> b;
blocksize = sqrt(a);
for (int i = 0; i < b; i++) {
int x, y;
cin >> x >> y;
z[x].push_back(y);
mark[i] = {x, y};
}
if (b == 0) {
cout << -1;
return 0;
}
dijkstra();
if (cnt.count({mark[1].first, 0})) {
cout << cnt[{mark[1].first, 0}];
} else {
cout << -1;
}
return 0;
}
