// cre: Nguyen Ngoc Hung - Train VOI 2024 :>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <unordered_set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#include <cstring>
#include <unordered_map>
#include <cmath>
#include <array>
#include <cassert>
#include <random>
using namespace std;
#define SORT_UNIQUE(v) sort(ALL(v)),(v).resize(unique(ALL(v))-(v).begin())
#define __nnhzzz__ signed main()
#define BIT(i,j) ((i>>j)&1LL)
#define MASK(i) (1LL<<i)
#define RALL(x) (x).rbegin(),(x).rend()
#define ALL(x) (x).begin(),(x).end()
#define SZ(x) (int)(x).size()
#define fi first
#define se second
#define INF 0x3f3f3f3f
#define ll long long
#define ull unsigned long long
#define ld long double
#define vi vector<int>
#define vvi vector<vi>
#define vvvi vector<vvi>
#define vvvvi vector<vvvi>
#define pii pair<int,int>
#define vpii vector<pii>
#define RESET(x,val) memset((x),(val),sizeof(x))
#define REPDIS(i,be,en,j) for(int i = (be); i<=(en); i+=j)
#define REPD(i,be,en) for(int i = (be); i>=(en); i--)
#define REP(i,be,en) for(int i = (be); i<=(en); i++)
//-------------------------------------------------------------//
const int oo = 1e9,LOG = 20,MAXN = 5e5+7,N = 1e2+3;
const int MOD = 1e9+7,MOD1 = 1e9+207,MOD2 = 1e9+407,MOD3 = 998244353;
//-------------------------------------------------------------//
template<typename T1, typename T2> bool mini(T1 &a, T2 b){if(a>b){a=b;return true;}return false;}
template<typename T1, typename T2> bool maxi(T1 &a, T2 b){if(a<b){a=b;return true;}return false;}
template<typename T> T gcd(T a, T b) { while(b) { a %= b; swap(a,b); } return a; }
template<typename T> T lcm(T a, T b) { return a/gcd(a,b)*b; }
/*
----------------------------------------------------------------
END OF TEMPLATE
----------------------------------------------------------------
Nguyen Ngoc Hung - nnhzzz
Training for VOI24 gold medal
----------------------------------------------------------------
*/
int dis[MAXN];
vi adj[MAXN];
void solve(){
int st,en,n,m; cin >> n >> m;
REP(i,0,m-1){
int b,p; cin >> b >> p;
if(i==0){
st = b;
}
if(i==1){
en = b;
}
adj[b].push_back(p);
}
memset(dis,0x3f3f3f3f,sizeof dis);
priority_queue<pii,vpii,greater<pii>> heap;
heap.emplace(0,st); dis[st] = 0;
while(SZ(heap)!=0){
auto [ww,u] = heap.top(); heap.pop();
if(dis[u]!=ww){
continue;
}
for(auto l:adj[u]){
for(int i = 1; u+(l*i)<n; i++){
int v = u+(l*i);
if(mini(dis[v],dis[u]+i)){
heap.emplace(dis[v],v);
}
}
for(int i = 1; u-(l*i)>=0; i++){
int v = u-(l*i);
if(mini(dis[v],dis[u]+i)){
heap.emplace(dis[v],v);
}
}
}
}
int res = dis[en];
if(res==0x3f3f3f3f){
res = -1;
}
cout << res;
}
__nnhzzz__{
ios::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
#define name "test"
if(fopen(name".inp","r")){
freopen(name".inp","r",stdin);
freopen(name".out","w",stdout);
}
#define name1 "nnhzzz"
if(fopen(name1".inp","r")){
freopen(name1".inp","r",stdin);
freopen(name1".out","w",stdout);
}
int test = 1;
while(test--){
solve();
}
cerr << "\nTime elapsed: " << 1000*clock()/CLOCKS_PER_SEC << "ms\n";
return 0;
}
