#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 = 3005;
int dis1[N][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 <= 2000; j++) {
dis1[i][j] = -1;
}
}
deque<pair<int, int>> q;
q.push_back({ b0, p0 });
dis1[b0][p0] = 0;
while (!q.empty()) {
int v = q.front().first;
int p = q.front().second;
q.pop_front();
if (v - p >= 0) {
if (dis1[v - p][p] == -1) {
dis1[v - p][p] = dis1[v][p] + 1;
q.push_back({ v - p, p });
}
}
if (v + p < n) {
if (dis1[v + p][p] == -1) {
dis1[v + p][p] = dis1[v][p] + 1;
q.push_back({ v + p, p });
}
}
for (int pp : doge[v]) {
if (dis1[v][pp] == -1) {
dis1[v][pp] = dis1[v][p];
q.push_front({ v, pp });
}
}
}
cout << dis1[b1][p1];
}
int main() {
migmig;
int T = 1;
//cin >> T;
while (T--) {
solve();
}
}
