#ifdef LOCAL
#include "local.h"
#else
#include <bits/stdc++.h>
#define debug(...)
#define DB(...)
#endif
using namespace std;
const bool __initialization = []() {
cin.tie(nullptr)->sync_with_stdio(false);
#define TASK "empty"
if (fopen(TASK".inp", "r")) {
(void)(!freopen(TASK".inp", "r", stdin));
(void)(!freopen(TASK".out", "w", stdout)); }
cout << setprecision(12) << fixed;
return true;
}();
using ll = long long;
using ld = long double;
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define For(i, l, r) for (int i = (l); i <= (r); ++i)
#define Ford(i, r, l) for (int i = (r); i >= (l); --i)
#define Rep(i, n) For (i, 0, (n) - 1)
#define Repd(i, n) Ford (i, (n) - 1, 0)
#define Fe(...) for (auto __VA_ARGS__)
template <class C> inline int isz(const C& c) { return static_cast<int>(c.size()); }
template <class T> inline bool chmin(T& a, const T& b) { return (a > b) ? a = b, true : false; }
template <class T> inline bool chmax(T& a, const T& b) { return (a < b) ? a = b, true : false; }
constexpr ld eps = 1e-9;
constexpr int inf = 1e9;
constexpr ll linf = 1e18;
// =============================================================================
constexpr int maxn = 3e4 + 5;
struct Link {
int v, c;
};
struct Data {
int u, du;
bool operator<(const Data& o) const {
return du > o.du;
}
};
int len, n;
int a[maxn], pw[maxn];
vector<Link> g[maxn];
int d[maxn];
priority_queue<Data> q;
void Dijkstra() {
memset(d, 0x3f, sizeof(int) * n);
d[0] = 0;
q.push({0, d[0]});
while (isz(q)) {
auto [u, du] = q.top(); q.pop();
if (du != d[u]) continue;
Fe (&[v, c] : g[u]) {
if (chmin(d[v], d[u] + c)) {
q.push({v, d[v]});
}
}
}
}
signed main() {
cin >> len >> n;
Rep (i, n) cin >> a[i] >> pw[i];
Rep (i, n) {
Rep (j, n) {
if (i == j) continue;
int dist = abs(a[i] - a[j]);
if (dist % pw[i] == 0) {
g[i].push_back({j, dist / pw[i]});
}
}
}
Dijkstra();
int ans = d[1];
if (ans >= inf) ans = -1;
cout << ans << '\n';
}
