#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
#pragma GCC optimize("O3")
#pragma GCC optimization("Ofast,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#define ll long long
//#define smid (nl + nr) / 2
//#define lc id * 2
//#define rc id * 2 + 1
#define migmig std::ios_base::sync_with_stdio(false);std::cin.tie(NULL);std::cout.tie(NULL);
template <class T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag,tree_order_statistics_node_update>;
const int N = 30005;
bitset<N>mark[N];
vector<int> doge[N];
void solve() {
int n, m; cin >> n >> m;
int b0, p0, b1, p1;
for (int i = 0; i < m; i++) {
int b, p; cin >> b >> p;
doge[b].push_back(p);
if (i == 1) {
b1 = b;
p1 = p;
}
if (i == 0) {
b0 = b;
p0 = p;
}
}
for (int i = 0; i <= n; i++) {
for (int j = 0; j <= 30000; j++) {
mark[i][j] = 0;
}
}
deque<array<int, 3>> q;
q.push_back({ 0, b0, p0 });
mark[b0][p0] = 1;
while (!q.empty()) {
int d = q.front()[0];
int v = q.front()[1];
int p = q.front()[2];
q.pop_front();
if (v == b1) {
cout << d; return;
}
if (v - p >= 0) {
if (!mark[v - p][p]) {
q.push_back({ d + 1, v - p, p });
mark[v - p][p] = 1;
}
}
if (v + p < n) {
if (!mark[v + p][p]) {
q.push_back({ d + 1, v + p, p });
mark[v + p][p] = 1;
}
}
for (int pp : doge[v]) {
if (!mark[v][pp]) {
q.push_front({ d, v, pp });
mark[v][pp] = 1;
}
}
doge[v].clear();
}
cout << -1;
}
int main() {
migmig;
int T = 1;
//cin >> T;
while (T--) {
solve();
}
}
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