#pragma GCC optimize("Ofast,O2,O3,unroll-loops")
#pragma GCC target("avx2")
#include <bits/stdc++.h>
using namespace std;
void debug_out() { cerr << endl; }
template<typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
cerr << "[" << H << "]";
debug_out(T...);
}
#ifdef dddxxz
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif
#define SZ(s) ((int)s.size())
#define all(x) (x).begin(), (x).end()
#define lla(x) (x).rbegin(), (x).rend()
#define bpc(x) __builtin_popcount(x)
#define bpcll(x) __builtin_popcountll(x)
clock_t startTime;
double getCurrentTime() {
return (double) (clock() - startTime) / CLOCKS_PER_SEC;
}
#define MP make_pair
typedef long long ll;
mt19937 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
const double eps = 0.000001;
const int MOD = 1e9 + 7;
const int INF = 1000000101;
const long long LLINF = 1223372000000000555;
const int N = 30001;
const int M = 1111;
bitset<30001> bit[30000];
vector<pair<int, int>> g[N];
int b[N], p[N];
int d[N];
bool processed[N];
void solve(int TC) {
int n, m;
cin >> n >> m;
for (int i = 0; i < m; i++){
cin >> b[i] >> p[i];
bit[b[i]].set(p[i]);
}
for (int i = 0; i < m; i++){
int j = b[i] + p[i], cnt = 1;
while (j < n){
g[b[i]].emplace_back(j, cnt);
if (bit[j][p[i]]) break;
cnt++;
j += p[i];
}
j = b[i] - p[i], cnt = 1;
while (j >= 0){
g[b[i]].emplace_back(j, cnt);
if (bit[j][p[i]]) break;
cnt++;
j -= p[i];
}
}
fill(d, d + n, INF);
d[b[0]] = 0;
priority_queue<pair<int, int>> pq;
pq.push(MP(0, b[0]));
while (!pq.empty()){
int v = pq.top().second;
pq.pop();
if (processed[v]) continue;
processed[v] = true;
for (auto [u, w] : g[v]){
if (d[u] > d[v] + w){
d[u] = d[v] + w;
pq.push(MP(-d[u], u));
}
}
}
cout << (d[b[1]] != INF ? d[b[1]] : -1);
}
int main() {
startTime = clock();
ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
#ifdef dddxxz
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
int TC = 1;
// cin >> TC;
for (int test = 1; test <= TC; test++) {
//debug(test);
//cout << "Case #" << test << ": ";
solve(test);
}
#ifdef dddxxz
cerr << endl << "Time: " << int(getCurrentTime() * 1000) << " ms" << endl;
#endif
return 0;
}
