#pragma GCC optimize("Ofast,unroll-all-loops")
#pragma GCC target("avx2,bmi,bmi2,popcnt,lzcnt,abm,sse4.2")
#include <bits/stdc++.h>
using namespace std;
#define int uint16_t
const int MAXM = 3e4+5, MAXN = 3e4+5;
int n, m, secloc, p[MAXM];
vector<int> a(MAXN, UINT16_MAX);
queue<int> b[MAXN];
struct triple {
int first, second;
char third;
triple() : first(0), second(0), third(0) {}
triple(int a, int b, char c) :first(a), second(b), third(c) {}
};
queue<triple> curr, oth;
signed main() {
cin.tie(0)->sync_with_stdio(0);
cin >> n >> m;
{
int tmp;
cin >> tmp >> p[0];
curr.push({tmp, p[0], 3});
b[tmp].push(0);
}
{
int tmp;
cin >> tmp >> p[1];
secloc = tmp;
b[tmp].push(1);
}
for(int i = 2; i < m; i++) {
int tmp;
cin >> tmp >> p[i];
b[tmp].push(i);
}
int curval = 0;
while(!curr.empty()) {
auto cur = curr.front(); curr.pop();
if(a[cur.first] == UINT16_MAX) {
set<int> ss;
while(!b[cur.first].empty()) {
int x = b[cur.first].front();
b[cur.first].pop();
if(p[x] != cur.second)
ss.insert(x);
}
for(auto x:ss) {
curr.push({cur.first, p[x], 3});
}
}
a[cur.first] = min(a[cur.first], curval);
if(cur.first >= cur.second && cur.third & 1)
oth.push({cur.first - cur.second, cur.second, 1});
if(cur.first < n - cur.second && cur.third & 2)
oth.push({cur.first + cur.second, cur.second, 2});
if(curr.empty()) {
swap(curr, oth);
curval ++;
}
}
cout << (a[secloc] == UINT16_MAX ? -1 : a[secloc]) << '\n';
}
