#include <bits/stdc++.h>
#define quan160607 "BoundSeq"
#define fi first
#define se second
#define pii pair<int, int>
#define piii pair<int, pii>
#define pb push_back
using namespace std;
typedef long long ll;
const ll oo = 1e18;
const int inf = 2e9;
const int nmax = 2000;
void start()
{
// freopen(quan160607".inp","r",stdin);
// freopen(quan160607".out","w",stdout);
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
}
int n, m;
int b[nmax], a[nmax], p[nmax], dp[nmax][nmax];
vector<int> v[nmax];
priority_queue<piii, vector<piii>, greater<piii>> q;
bool ck[nmax];
bool minimize(int &a, int b)
{
if(a > b)
{
a = b;
return true;
} else return false;
}
void dij()
{
for(int i = 0; i <= n; i++)
{
for(int j = 0; j <= n; j++)
dp[i][j] = inf;
}
// cout << "kaka\n";
for(auto x : v[b[0]])
{
dp[b[0]][p[x]] = 0;
q.push({dp[b[0]][p[x]], {b[0], x}});
}
// cout << b[0] << "\n";
while(!q.empty())
{
piii temp = q.top();
q.pop();
int u = temp.se.fi, pk = p[temp.se.se];
// cout << u << " " << pk << "\n";
if(temp.fi != dp[u][pk])
continue;
// cout << "kaka\n";
for(auto x : v[u])
{
// cout << p[x] << "\n";
if(p[x] > n) continue;
for(int i = u + p[x]; i < n; i += p[x]) {
// cout << i << " ";
if(minimize(dp[i][p[x]], dp[u][pk] + (i - u) / p[x]))
q.push({dp[i][p[x]], {i, x}});
}
for(int i = u - p[x]; i >= 0; i -= p[x]) {
if(minimize(dp[i][p[x]], dp[u][pk] + (u - i) / p[x]))
q.push({dp[i][p[x]], {i, x}});
}
}
}
}
int main()
{
start();
cin >> n >> m;
for(int i = 0; i < m; i++)
{
cin >> b[i] >> p[i];
v[b[i]].push_back(i);
}
dij();
int ans = inf;
for(int j = 0; j <= n; j++)
ans = min(ans, dp[b[1]][j]);
cout << (ans == inf ? -1 : ans);
}
