/// In The Name Of God
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include <bits/stdc++.h>
#define f first
#define s second
#define pb push_back
#define pp pop_back
#define mp make_pair
#define sz(x) (int)x.size()
#define sqr(x) ((x) * 1ll * (x))
#define all(x) x.begin(), x.end()
#define Kazakhstan ios_base :: sync_with_stdio(0), cin.tie(0), cout.tie(0);
#define nl '\n'
#define ioi exit(0);
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
const int N = (int)3e4 + 7;
const int inf = (int)1e9 + 7;
const int mod = (int)1e9 + 7;
const ll linf = (ll)1e18 + 7;
const int dx[] = {-1, 0, 1, 0, 1, -1, -1, 1};
const int dy[] = {0, 1, 0, -1, 1, -1, 1, -1};
using namespace std;
int n, m;
int b[N], p[N];
vector < pair <int, int> > g[N];
ll d[N];
int main() {
#ifdef IOI2018
freopen ("in.txt", "r", stdin);
#endif
Kazakhstan
cin >> n >> m;
for (int i = 1; i <= m; i++) {
cin >> b[i] >> p[i];
}
for (int i = 1; i <= m; i++) {
for (int j = 1; j <= min(5000, m); j++) {
if (i == j) continue;
if (abs(b[i] - b[j]) % p[i] == 0) {
g[i].pb({j, abs(b[i] - b[j]) / p[i]});
}
}
}
set < pair <ll, int > > st;
memset(d, 0x3f, sizeof(d));
st.insert({0, 1});
d[1] = 0;
while (sz(st)) {
pair <ll, int> v = *st.begin();
st.erase(st.begin());
for (auto to : g[v.s]) {
if (d[to.f] > v.f + to.s) {
st.erase({d[to.f], to.f});
d[to.f] = v.f + to.s;
st.insert({d[to.f], to.f});
}
}
}
if (d[2] >= linf) cout << -1;
else cout << d[2];
ioi
}
