#include <bits//stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef tree<long long, null_type, less<long long>, rb_tree_tag,
tree_order_statistics_node_update>
ordered_set;
typedef tree<long long, null_type, less_equal<long long>, rb_tree_tag,
tree_order_statistics_node_update>
ordered_multiset;
#define ll long long
#define ld long double
#define iloop(m, h) for (auto i = m; i != h; i += (m < h ? 1 : -1))
#define jloop(m, h) for (auto j = m; j != h; j += (m < h ? 1 : -1))
#define kloop(m, h) for (auto k = m; k != h; k += (m < h ? 1 : -1))
#define lloop(m, h) for (auto l = m; l != h; l += (m < h ? 1 : -1))
#define iloop_(m, h, g) for (auto i = m; i < h; i += g)
#define jloop_(m, h, g) for (auto j = m; j < h; j += g)
#define kloop_(m, h, g) for (auto k = m; k < h; k += g)
#define lloop_(m, h, g) for (auto l = m; l < h; l += g)
#define getchar_unlocked _getchar_nolock // comment before submission
#define pll pair<ll, ll>
#define plll pair<ll, pll>
#define pllll pair<pll, pll>
#define vll vector<ll>
#define qll queue<ll>
#define dll deque<ll>
#define pqll priority_queue<ll>
#define gll greater<ll>
#define INF 1000000000000000
#define MOD1 1000000007
#define MOD2 998244353
#define MOD3 1000000009
mt19937 rng(chrono::system_clock::now().time_since_epoch().count());
ll n, m, sx, ex, blk = 175;
vector<ll> av[30005];
ll dst[30005];
bool vis[30005][180];
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin >> n >> m;
ll t1, t2;
iloop(0, m) {
cin >> t1 >> t2;
if (i == 0) sx = t1;
if (i == 1) ex = t1;
av[t1].push_back(t2);
}
priority_queue<pll, vector<pll>, greater<pll>> pq;
pq.push({0, sx});
iloop(0, n) dst[i] = INF;
dst[sx] = 0;
memset(vis, 0, sizeof(vis));
while (pq.size()) {
pll nd = pq.top();
pq.pop();
if (nd.first > dst[nd.second]) continue;
for (auto it : av[nd.second]) {
t1 = 0;
for (ll ci = nd.second + it; ci < n; ci += it) {
t1++;
if (nd.first + t1 < dst[ci]) {
dst[ci] = nd.first + t1;
pq.push({dst[ci], ci});
}
if (it < blk && vis[ci][it]) break;
if (it < blk) vis[ci][it] = 1;
}
t1 = 0;
for (ll ci = nd.second - it; ci >= 0; ci -= it) {
t1++;
if (nd.first + t1 < dst[ci]) {
dst[ci] = nd.first + t1;
pq.push({dst[ci], ci});
}
if (it < blk && vis[ci][it]) break;
if (it < blk) vis[ci][it] = 1;
}
}
}
cout << (dst[ex] < INF ? dst[ex] : -1);
}
