#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 = 3e4 + 5;
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], pp[nmax];
bool ck[nmax];
vector<int> bp[nmax];
const int block = 150;
int dp[nmax][block + 5];
priority_queue<piii, vector<piii>, greater<piii>> q;
bool minimize(int &a, int b)
{
if(a > b)
{
a = b;
return true;
} else return false;
}
void solve_dij()
{
for(int i = 0; i <= n; i++)
{
for(int j = 0; j <= block; j++)
dp[i][j] = inf;
}
dp[0][0] = 0;
q.push({dp[0][0], {0, 0}}); // dp[u][x], u, x
// dp[i][0] : đến đỉnh i từ 1 đỉnh khác bằng năng lượng > block
// dp[i][x] : đến đỉnh i từ 1 đỉnh khác bằng năng lượng x <= block
// cout << dp[1][0] << "\n";
piii temp;
while(!q.empty())
{
temp = q.top();
q.pop();
int u = temp.se.fi;
if(temp.fi != dp[u][temp.se.se])
continue;
if(ck[u] == false)
{
for(auto x : bp[u])
{
if(x <= block)
{
if(minimize(dp[u][x], dp[u][temp.se.se]))
q.push({dp[u][x], {u, x}});
}
else
{
if(minimize(dp[u][0], dp[u][temp.se.se]))
q.push({dp[u][0], {u, 0}});
}
}
ck[u] = true;
}
if(temp.se.se > 0)
{
int x = temp.se.se;
if(u - x >= 1){
if(minimize(dp[u - x][x], dp[u][x] + 1))
q.push({dp[u - x][x], {u - x, x}});
}
if(u + x <= n){
if(minimize(dp[u + x][x], dp[u][x] + 1))
q.push({dp[u + x][x], {u + x, x}});
}
}
else
{
// cout << "kaka " << u << "\n";
for(int x : bp[u])
{
if(x > block)
{
for(int i = u + x; i <= n; i += x)
{
if(minimize(dp[i][0], dp[u][0] + (i - u) / x))
q.push({dp[i][0], {i, 0}});
}
for(int i = u - x; i >= 1; i -= x)
{
if(minimize(dp[i][0], dp[u][0] + (u - i) / x))
q.push({dp[i][0], {i, 0}});
}
}
}
}
}
}
int main()
{
start();
cin >> n >> m;
for(int i = 1; i <= m; i++)
{
cin >> b[i] >> p[i];
bp[b[i]].push_back(p[i]);
}
solve_dij();
int ans = inf;
ans = min(ans, dp[1][0]);
// for(int j = 0; j <= block; j++)
// {
// if(dp[1][j] != inf)
// cout << j << " " << dp[1][j] << "\n";
// }
for(int j = 1; j <= block; j++)
ans = min(ans, dp[1][j]);
cout << (ans == inf ? -1 : ans);
// cout << dp[1][1];
}
