#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;
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);
}
deque<array<int, 3>> dq;
map<pii, int> dp;
dq.push_back({0, d0.first, d0.second});
dp[d0] = 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.count({i, a}) || 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.count({j, p}) || d + 1 < dp[{j, p}]){
dp[{j, p}] = d + 1;
dq.push_back({d + 1, j, p});
}
}
}
if(!dp.count(d1)) cout << "-1\n";
else cout << dp[d1] << '\n';
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
int t = 1;
// cin >> t;
while (t--) solve();
}
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