#include <bits/stdc++.h>
#define ll long long
#define fi first
#define se second
#define sz(x) (int)x.size()
#define ALL(v) v.begin(),v.end()
#define MASK(k) (1LL << (k))
#define BIT(x, i) (((x) >> (i)) & 1)
#define oo (ll)1e18
#define INF (ll)1e9
#define MOD (ll)(998244353)
using namespace std;
template<class T1, class T2>
bool maximize(T1 &a, T2 b){if(a < b){a = b; return true;} return false;}
template<class T1, class T2>
bool minimize(T1 &a, T2 b){if(a > b){a = b; return true;} return false;}
template<class T1, class T2>
void add(T1 &a, T2 b){a += b; if(a >= MOD) a -= MOD;}
template<class T1, class T2>
void sub(T1 &a, T2 b){a -= b; if(a < 0) a += MOD;}
template<class T>
void cps(T &v){sort(ALL(v)); v.resize(unique(ALL(v)) - v.begin());}
const int MAX = 3e4 + 10;
int N, M, S, T;
vector<int> adj[MAX];
int f[MAX];
void solve(){
cin >> N >> M;
for(int i=0; i<M; i++){
int p, d;
cin >> p >> d;
adj[p].push_back(d);
if(i == 0){
S = p;
}
if(i == 1){
T = p;
}
}
memset(f, 0x3f, sizeof f);
f[S] = 0;
priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int>>> pq;
pq.push(make_pair(0, S));
while(sz(pq)){
int u = pq.top().se;
int kc = pq.top().fi;
pq.pop();
if(kc > f[u]) continue;
for(int d: adj[u]){
int cnt = 0;
for(int i=u+d; i<N; i+=d){
cnt++;
if(minimize(f[i], f[u] + cnt)){
pq.push(make_pair(f[i], i));
}
}
cnt = 0;
for(int i=u-d; i>=0; i-=d){
cnt++;
if(minimize(f[i], f[u] + cnt)){
pq.push(make_pair(f[i], i));
}
}
}
}
cout << (f[T] >= INF ? -1 : f[T]) << '\n';
}
int main(){
ios_base::sync_with_stdio(0); cin.tie(0);
// freopen("numtable.inp","r",stdin);
// freopen("numtable.out","w",stdout);
solve();
return 0;
}
