#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <bits/stdc++.h>
#define fi first
#define se second
#define pb push_back
#define sz(a) (int)a.size()
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define ff(i,a,b) for(int i=a;i<=b;i++)
#define fb(i,b,a) for(int i=b;i>=a;i--)
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef pair<int,int> pii;
const int maxn = 100005;
const int inf = 1e9 + 5;
template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
// os.order_of_key(k) the number of elements in the os less than k
// *os.find_by_order(k) print the k-th smallest number in os(0-based)
int n, m;
set<int> skok[maxn];
vector<pii> g[maxn];
bool pripada(int X) {
return (X >= 0 && X < n);
}
ll dist[maxn];
void dij(int s) {
ff(i,0,n - 1)dist[i] = inf;
priority_queue<pii, vector<pii>, greater<pii>> pq;
pq.push({dist[s] = 0, s});
while(!pq.empty()) {
pii v = pq.top();
pq.pop();
if(dist[v.se] < v.fi)continue;
for(auto c : g[v.se]) {
int u = c.fi;
int w = c.se;
if(dist[u] > w + v.fi) {
pq.push({dist[u] = w + v.fi, u});
}
}
}
}
int main()
{
ios::sync_with_stdio(false);
cout.tie(nullptr);
cin.tie(nullptr);
cin >> n >> m;
int start = 0, end = 0;
ff(i,0,m - 1) {
int X, Y;
cin >> X >> Y;
if(i != 1)skok[X].insert(Y);
if(i == 0)start = X;
if(i == 1)end = X;
}
ff(i,0,n - 1) {
for(auto c : skok[i]) {
int j = 1;
while(1) {
int nw = i + j * c;
if(!pripada(nw))break;
g[i].pb({nw, j});
if(skok[nw].count(c))break;
j += 1;
}
}
}
ff(i,0,n - 1) {
for(auto c : skok[i]) {
int j = 1;
while(1) {
int nw = i - j * c;
if(!pripada(nw))break;
g[i].pb({nw, j});
if(skok[nw].count(c))break;
j += 1;
}
}
}
dij(start);
cout << (dist[end] == inf ? -1 : dist[end]) << endl;
return 0;
}
/**
5 3
0 2
1 1
4 1
// probati bojenje sahovski ili slicno
**/
