#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
#include <bitset>
using namespace std;
const int MAXN = 30100;
const int SQN = 100;
const int INF = 1e9 + 177013;
pair<int, int> dogs[MAXN];
vector<int> jlen[MAXN];
struct modmin {
void resize(int n) {
sz = n;
for (int i = 0; i < n; ++i)
active.insert(i);
pivots = {{-INF, 10}, {INF, 10}};
}
void insert_pivot(int x, int vl) {
auto it = active.lower_bound(x);
if (it != active.end())
best.erase({get_value(*it), *it});
if (it != active.begin())
best.erase({get_value(*prev(it)), *prev(it)});
pivots.insert(lower_bound(pivots.begin(), pivots.end(), pair<int, int>{x, -1}), {x, vl});
if (it != active.end())
best.insert({get_value(*it), *it});
if (it != active.begin())
best.insert({get_value(*prev(it)), *prev(it)});
}
void deactivate(int x) {
int vl = get_value(x);
auto it = active.find(x);
if (best.find({vl, x}) != best.end()) {
best.erase({vl, x});
if (next(it) != active.end())
best.insert({get_value(*next(it)), *next(it)});
if (it != active.begin())
best.insert({get_value(*prev(it)), *prev(it)});
}
active.erase(it);
}
pair<int, int> get_min() {
if (best.empty())
return {INF, INF};
return *best.begin();
}
private:
int sz;
set<pair<int, int>> pivots;
set<int> active;
set<pair<int, int>> best;
int get_value(int x) {
auto it = pivots.lower_bound({x, -1});
return min(get_transit(it, x), get_transit(prev(it), x));
}
int get_transit(set<pair<int, int>>::iterator it, int x) {
return abs(x - it->first) + it->second;
}
};
bitset<MAXN> used;
int dist_long[MAXN];
modmin dist_short[SQN][MAXN / SQN];
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
int n, m;
cin >> n >> m;
for (int i = 0; i < m; ++i) {
cin >> dogs[i].first >> dogs[i].second;
jlen[dogs[i].first].push_back(dogs[i].second);
}
int s = dogs[0].first;
int t = dogs[1].first;
for (int i = 0; i < n; ++i)
dist_long[i] = INF;
dist_long[s] = 0;
for (int d = 1; d < SQN; ++d) {
for (int md = 0; md < n && md < d; ++md) {
dist_short[d][md].resize((n - md - 1) / d + 1);
}
}
set<pair<int, int>> rdist_short;
int T = n;
while (T--) {
//cout << "A" << endl;
pair<int, int> opt = {INF, INF};
for (int i = 0; i < n; ++i) {
if (!used[i])
opt = min(opt, {dist_long[i], i});
}
if (rdist_short.size()) {
opt = min(opt, *rdist_short.begin());
}
auto [dist, v] = opt;
if (dist >= INF)
break;
dist_long[v] = dist;
used[v] = 1;
for (int d = 1; d < SQN; ++d) {
int md = v % d;
pair<int, int> cval = dist_short[d][md].get_min();
cval.second = md + d * cval.second;
rdist_short.erase(cval);
dist_short[d][md].deactivate(v / d);
pair<int, int> ncval = dist_short[d][md].get_min();
ncval.second = md + d * ncval.second;
rdist_short.insert(ncval);
}
for (auto p : jlen[v]) {
if (p >= SQN) {
for (int k = 1; p * k <= v; ++k)
dist_long[v - p * k] = min(dist_long[v - p * k], dist + k);
for (int k = 1; v + p * k < n; ++k)
dist_long[v + p * k] = min(dist_long[v + p * k], dist + k);
} else {
int md = v % p;
pair<int, int> cval = dist_short[p][md].get_min();
cval.second = md + p * cval.second;
rdist_short.erase(cval);
dist_short[p][md].insert_pivot(v / p, dist);
pair<int, int> ncval = dist_short[p][md].get_min();
ncval.second = md + p * ncval.second;
rdist_short.insert(ncval);
}
}
}
if (dist_long[t] >= INF)
cout << "-1\n";
else
cout << dist_long[t] << '\n';
}
