#include<bits/stdc++.h>
#define fo(i,d,c) for(int i=d;i<=c;i++)
#define fod(i,c,d) for(int i=c;i>=d;i--)
#define maxn 30010
#define N 1010
#define fi first
#define se second
#define pb emplace_back
#define en cout<<"\n";
#define int long long
#define inf (int)1e18
#define pii pair<int,int>
#define vii vector<pii>
#define lb(x) x&-x
#define bit(i,j) ((i>>j)&1)
#define offbit(i,j) (i^(1<<j))
#define onbit(i,j) (i|(1<<j))
#define vi vector<int>
template <typename T1, typename T2> bool minimize(T1 &a, T2 b){if (a > b) {a = b; return true;} return false;}
template <typename T1, typename T2> bool maximize(T1 &a, T2 b){if (a < b) {a = b; return true;} return false;}
using namespace std;
int n,m;
pii a[maxn];
int d[maxn];
vii ke[maxn];
void dij(int inp)
{
fo(i,1,m) d[i] = inf;
d[inp] = 0;
priority_queue<pii,vii,greater<pii>> q;
q.push({0,inp});
while(q.size())
{
auto [du,u] = q.top();
q.pop();
if(du != d[u]) continue;
for(auto [v,w] : ke[u])
{
if(d[v] > d[u] + w)
{
d[v] = d[u] + w;
q.push({d[v],v});
}
}
}
}
main()
{
#define name "TASK"
if(fopen(name".inp","r"))
{
freopen(name".inp","r",stdin);
freopen(name".out","w",stdout);
}
ios_base::sync_with_stdio(false);cin.tie(NULL);
cin >> n >> m;
fo(i,1,m) cin >> a[i].fi >> a[i].se;
fo(i,1,m) fo(j,1,m) if(i != j)
{
int val = abs(a[i].fi - a[j].fi);
if(val % a[i].se == 0)
ke[i].pb(j,val / a[i].se);
}
dij(1);
cout << (d[2] >= inf ? -1 : d[2]);
}
