//Sylwia Sapkowska
#include <bits/stdc++.h>
#pragma GCC optimize("O3", "unroll-loops")
using namespace std;
void __print(int x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << "'" << x << "'";}
void __print(const char *x) {cerr << '"' << x << '"';}
void __print(const string &x) {cerr << '"' << x << '"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ", "; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? ", " : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifdef LOCAL
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif
// #define int long long
typedef pair<int, int> T;
const int oo = 1e9+7, K = 30;
const int mod = 998244353;
const int S = 2;
void solve(){
int n, m; cin >> n >> m;
vector<vector<T>>g(n*S+2);
auto ch = [&](int v, int j = 0){return v*S+j;};
vector<vector<int>>doge(n+1);
int start, kon;
for (int i = 0; i<m; i++){
int v, j; cin >> v >> j;
if (i == 0) start = ch(v);
if (i == 1) kon = ch(v);
// if (j >= n) continue;
if (j < S) g[ch(v)].emplace_back(ch(v, j), 0);
else doge[v].emplace_back(j);
}
vector<int>dist(n*S+2, oo);
set<T>s;
debug(start, kon);
s.insert({0, start});
dist[start] = 0;
while ((int)s.size()){
auto [d, v] = *s.begin();
s.erase(s.begin());
debug(v);
if (dist[v] < d) continue;
//dlugie skoki
debug(v, doge[v]);
auto popraw = [&](int what, int from, int c){
debug(what, from, c);
if (dist[what] > dist[from] + c){
dist[what] = dist[from] + c;
s.insert({dist[what], what});
}
};
if (v%S == 0){
int vv = v/S;
for (auto j: doge[vv]){
for (int u = vv-j, k = 1; u >= 0; u-=j, k++){
popraw(ch(u), v, k);
}
for (int u = vv+j, k = 1; u<n; u+=j, k++){
popraw(ch(u), v, k);
}
}
}
//jakies inne losowe
for (auto [x, c]: g[v]){
popraw(x, v, c);
}
//krotkie skoki
int j = v%S;
if (j != 0){
int pos = v-v%S;
popraw(pos, v, 0);
int u = pos/S;
debug(u, j);
if (u >= j){
popraw(ch(u-j, j), v, 1);
}
if (u + j < n){
popraw(ch(u+j, j), v, 1);
}
}
}
int ans = dist[kon];
if (ans == oo) ans = -1;
cout << ans << "\n";
}
int32_t main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int t = 1;
//cin >> t;
while (t--) solve();
return 0;
}
