#include<bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define pb push_back
#define ep emplace
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define uniquev(v) sort(all(v)), (v).resize(unique(all(v)) - (v).begin())
#define mem(f,x) memset(f , x , sizeof(f))
#define sz(x) (int32_t)(x).size()
#define __lcm(a, b) (1ll * ((a) / __gcd((a), (b))) * (b))
#define mxx *max_element
#define mnn *min_element
#define left Kurumi_Tokisaki
#define right Kei_Karuizawa
#define next Mai_Sakurajima
#define div Yume_Irido
#define prev Chizuru_Mizuhara
#define cntbit(x) __builtin_popcountll(x)
#define MASK(x) ( 1ll << (x) )
#define Yes cout << "Yes"
#define YES cout << "YES"
#define No cout << "No"
#define NO cout << "NO"
#define AA cout << "Alice"
#define BB cout << "Bob"
/// TASK
/// -----------------------------
#ifdef LMQZZZ
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 deb(x...) cerr << "[ in " <<__func__<< "() : line " <<__LINE__<< " ] : [ " << #x << " ] = [ "; _print(x); cerr << '\n';
#define TASK "C"
#else
#define deb(x...) 3326
#define TASK "Sky"
#endif
///------------------------------
void lmqzzz();
void init() {};
int32_t main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
if (fopen(TASK ".inp", "r")) {
freopen(TASK ".inp", "r", stdin);
freopen(TASK ".out", "w", stdout);
}
/// =================================
constexpr bool MULTITEST = 0;
/// ================================
init();
int32_t TT = 1;
if ( MULTITEST ) cin >> TT;
for(int32_t TTT = 1; TTT <= TT; TTT ++) {
lmqzzz();
cout << '\n';
}
}
template <class T> inline T min(const T &a, const T &b, const T &c) { return min(a, min(b, c)); }
template <class T> inline T max(const T &a, const T &b, const T &c) { return max(a, max(b, c)); }
template <class T, class U> inline bool mini(T &a, const U &b) { if (a > b) { a = b; return 1; } return 0; }
template <class T, class U> inline bool maxi(T &a, const U &b) { if (a < b) { a = b; return 1; } return 0; }
constexpr int16_t dr[] = {0, 0, -1, 1};
constexpr int16_t dc[] = {1, -1, 0, 0};
constexpr int64_t MOD = 1e9 + 7;
constexpr int32_t MAXN = 1e6 + 10;
int n, m;
int b[MAXN], p[MAXN], d[MAXN];
vector<int> adj[MAXN];
void lmqzzz() {
cin >> n >> m;
for (int i = 0; i < m; i ++) {
cin >> b[i] >> p[i];
adj[b[i]].eb(p[i]);
}
priority_queue<pair<int, int>> pq;
pq.ep(0, b[0]);
mem(d, 0x3f);
d[b[0]] = 0;
while (sz(pq)) {
int u = pq.top().se;
int du = -pq.top().fi;
pq.pop();
if (du != d[u]) continue;
if (u == b[1]) {
cout << du;
return;
}
for (int w : adj[u]) {
for (int next = u % w; next < n; next += w) {
if (mini(d[next], du + abs(next - u) / w)) {
pq.ep(-d[next], next);
}
}
}
}
cout << -1;
}
