#include <bits/stdc++.h>
#define FIXED_FLOAT(x) std::fixed <<std::setprecision(20) << (x)
#define all(v) (v).begin(), (v).end()
using namespace std;
#define forn(i,n) for (int i = 0; i < (n); ++i)
#define rforn(i, n) for(int i = (n) - 1;i >= 0;--i)
using ll = long long;
int mod = (ll)1e9 + 7;
#define PI acos(-1)
typedef pair<int, int> pairs;
const int INF = 1e9 + 1;
const int N = 2e5 + 100;
const double eps = 1e-7;
template <class T> using V = vector<T>;
template <class T> using VV = V<V<T>>;
template <typename XPAX>
void ckma(XPAX &x, XPAX y) {
x = (x < y ? y : x);
}
template <typename XPAX>
void ckmi(XPAX &x, XPAX y) {
x = (x > y ? y : x);
}
void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
void solve() {
int n, m;
cin >> n >> m;
VV<int> g(n);
V<int> f(m), p(m);
V<bool> mark(n, false);
V<int> vis(n, false);
forn(i, m) {
cin >> f[i] >> p[i];
g[f[i]].push_back(p[i]);
mark[f[i]] = 1;
}
int x;
V<int> dis(n, INF);
priority_queue<pairs, V<pairs>, greater<pairs>> q;
q.push({0, f[0]});
dis[f[0]] = 0;
while(q.size()) {
x = q.top().second;
int s = q.top().first;
if(x == f[1]) {
cout << dis[x] << '\n';
exit(0);
}
if(vis[x])continue;
vis[x] = 1;
q.pop();
for(auto c : g[x]) {
for(int i = x + c, cnt = 1;i < n;i += c, ++cnt) {
if(mark[i] && s + cnt < dis[i]) {
dis[i] = s + cnt;
q.push({dis[i], i});
}
}
for(int i = x - c, cnt =1;i >= 0;i -= c, cnt++) {
if(mark[i] && s + cnt < dis[i]) {
dis[i] = s + cnt;
q.push({dis[i], i});
}
}
}
}
//debug(dis);
cout << -1 << '\n';
}
void test_case() {
int t;
cin >> t;
forn(p, t) {
//cout << "Case #" << p + 1 << ": ";
solve();
}
}
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
solve();
}
