#include <bits/stdc++.h>
// #pragma GCC optimize("Ofast,no-stack-protector,unroll-loops")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
// #define int long long
#define pii pair<int, int>
#define ld long double
#define all(v) v.begin(), v.end()
using namespace std;
const int oo = 1e9 + 9;
const int MAX = 3e4 + 6;
int MOD = 998244353;
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
void solve(){
int n, m;
cin >> n >> m;
set<int> arr[n + 1];
pii d0, d1;
for(int i = 0; i < m; i++){
int x, p; cin >> x >> p;
if(i==0) d0 = {x, p};
if(i==1) d1 = {x, p};
arr[x].insert(p);
}
gp_hash_table<int, int> dp[n + 1];
deque<array<int, 3>> dq;
dq.push_back({0, d0.first, d0.second});
dp[d0.first][d0.second] = 0;
while(dq.size()){
auto [d, i, p] = dq.front();
dq.pop_front();
if(d != dp[i][p]) continue;
for(int a : arr[i]){
if(dp[i].find(a) == dp[i].end() || d < dp[i][a]){
dp[i][a] = d;
dq.push_front({d, i, a});
}
}
arr[i].clear();
for(int j = i - p; j <= i + p; j += 2 * p){
if(!(0 <= j && j < n)) continue;
if(dp[j].find(p) == dp[i].end() || d + 1 < dp[j][p]){
dp[j][p] = d + 1;
dq.push_back({d + 1, j, p});
}
}
}
if(dp[d1.first].find(d1.second) == dp[d1.first].end()) cout << "-1\n";
else cout << dp[d1.first][d1.second] << '\n';
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
int t = 1;
// cin >> t;
while (t--) solve();
}
