#include<bits/stdc++.h>
#define oo 1000000000
#define maxm 4000000
#define maxn 50005
#define block 220
using namespace std;
typedef long long ll;
int n, m;
int b[maxn], p[maxn];
vector < pair < int, int > > g[maxm];
int node(int u, int x) {return x * n + u;}
int inode(int x) {return x % n;}
int dnode(int x) {return (x - inode(x)) / n;}
int dist[maxm];
int dijkstra() {
priority_queue < pair < int, int > > p;
fill(dist, dist + maxm, oo);
dist[b[0]] = 0;
p.push({0, b[0]});
while (!p.empty()) {
int u = p.top().second; p.pop();
for (auto i : g[u]) {
int v = i.first, w = i.second;
if (dist[v] > dist[u] + w) {
// cerr << u << ' ' << v << ' ' << w << '\n';
dist[v] = dist[u] + w;
p.push({-dist[v], v});
}
}
}
//for (int i = 0; i < 25; ++i)
//cerr << i << ' ' << dist[i] << '\n';
return (dist[b[1]] != oo ? dist[b[1]] : -1);
}
int main(){
#define TASK "ABC"
//freopen(TASK".inp", "r", stdin); freopen(TASK".out", "w", stdout);
ios_base::sync_with_stdio(0);
cin >> n >> m;
for (int i = 0; i < m; ++i)
cin >> b[i] >> p[i];
for (int j = 1; j <= block; ++j)
for (int i = j; i < n; ++i)
g[node(i, j)].push_back({node(i - j, j), 1});
for (int j = 1; j <= block; ++j)
for (int i = 0; i + j < n; ++i)
g[node(i, j)].push_back({node(i + j, j), 1});
for (int i = 0; i < n; ++i)
for (int j = 1; j <= block; ++j)
g[node(i, j)].push_back({i, 0});
for (int i = 0; i < m; ++i)
if (p[i] <= block)
g[b[i]].push_back({node(b[i], p[i]), 0});
for (int i = 0; i < m; ++i)
if (p[i] > block)
for (int j = b[i] % p[i]; j < n; j += p[i])
if (i != j)
g[b[i]].push_back({j, abs(j - b[i]) / p[i]});
//for (int i = 0; i < 10; ++i)
// for (auto j : g[i])
// cerr << i << ' ' << j.first << ' ' << j.second << '\n';
cout << dijkstra();
return 0;
}
