#include <bits/stdc++.h>
using namespace std;
#define task "main"
#define F first
#define S second
#define ii pair<int, int>
#define il pair<int, long long>
#define li pair<long long, int>
#define FOR(i, a, b) for(int i = (a); i <= (b); ++i)
#define FOD(i, b, a) for(int i = (b); i >= (a); --i)
template <class T1, class T2>
bool maximize(T1 &a, T2 b){
if (a < b) {a = b; return true;}
return false;
}
template <class T1, class T2>
bool minimize(T1 &a, T2 b){
if (a > b) {a = b; return true;}
return false;
}
template <class T>
void printArr(T container, string separator = " ", string finish = "\n", ostream &out = cout){
for(auto item: container) out << item << separator;
out << finish;
}
const int MAX_N = (int)3e4 + 4;
int nNode, nDog;
map<int, bool> hadR[MAX_N], hadL[MAX_N];
vector<ii> adj[MAX_N];
long long dist[MAX_N];
void dijkstra(int s) {
memset(dist, 0x3f, sizeof(dist));
dist[s] = 0;
priority_queue<li, vector<li>, greater<li>> pq;
pq.push({0, s});
while (!pq.empty()) {
li t = pq.top();
pq.pop();
int u = t.S;
if (dist[u] != t.F) continue;
for (ii e : adj[u]) {
int v = e.F, w = e.S;
if (minimize(dist[v], dist[u] + w))
pq.push({dist[v], v});
}
}
}
void solve() {
cin >> nNode >> nDog;
int s = -1, e = -1;
FOR(i, 0, nDog - 1) {
int pos, r;
cin >> pos >> r;
if (i == 0) s = pos;
else if (i == 1) e = pos;
int a = pos, d = 0;
while (a + r < nNode) {
if (hadR[a][r]) break;
hadR[a][r] = 1;
adj[pos].push_back({a + r, ++d});
a += r;
}
a = pos, d = 0;
while (a - r >= 0) {
if (hadL[a][r]) break;
hadL[a][r] = 1;
adj[pos].push_back({a - r, ++d});
a -= r;
}
}
dijkstra(s);
cout << (dist[e] >= dist[nNode + 1] ? -1 : dist[e]);
}
int32_t main() {
if (fopen(task".inp", "r")) {
freopen(task".inp", "r", stdin);
freopen(task".out", "w", stdout);
}
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
bool multitest = 0;
int numTest = 1;
if (multitest) cin >> numTest;
while (numTest--) {
solve();
}
return 0;
}
/* Lak lu theo dieu nhac!!!! */
