#include<bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// using namespace __gnu_pbds;
// #pragma GCC target ("avx,avx2,fma")
// #pragma GCC optimize("Ofast")
// #pragma GCC optimize("unroll-loops")
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
#define SPEED ios_base::sync_with_stdio(false); cin.tie(0), cout.tie(0)
#define rall(v) (v).rbegin(),(v).rend()
#define all(v) (v).begin(),(v).end()
#define setp fixed<<setprecision
#define OK cerr<<"OK"<<endl<<flush
#define pii pair<int, int>
#define pll pair<ll, ll>
#define pb push_back
#define F first
#define S second
#define y0 jahdakdh
#define y1 jahsadakdakdh
#define endl '\n'
const ll MOD=1e9+7;
const ll mod=(1ll<<31)-1;
const ld eps=1e-8;
using namespace std;
mt19937 rng(std::chrono::system_clock::now().time_since_epoch().count());
int n, m, x[30005], pos[30005], pw[30005];
priority_queue<pii>q;
bool used[30005];
ll dis[30005];
vector<pii> g[30005];
int main()
{
SPEED;
memset(x, -1, sizeof(x));
cin>>n>>m;
for(int i=0; i<m; i++)
{
int b, p;
cin>>b>>p;
pos[i]=b;
pw[i]=p;
x[b]=i;
}
for(int i=0; i<m; i++)
{
for(int j=0; j<m; j++)
{
if(i==j) continue;
if(abs(pos[i]-pos[j])%pw[i]==0) g[i].pb({j, abs(pos[i]-pos[j])/pw[i]});
}
}
memset(dis, 63, sizeof(dis));
q.push({0,0});
dis[0]=0;
while(!q.empty())
{
int s=q.top().S;
q.pop();
if(used[s]) continue;
used[s]=1;
for(pii u : g[s])
{
if(dis[s]+u.S<dis[u.F])
{
dis[u.F]=dis[s]+u.S;
q.push({-dis[u.F], u.F});
}
}
}
if(dis[1]>1e9) cout<<-1;
else cout<<dis[1];
return 0;
}
/*
5 3
0 2
1 1
4 1
*/
