#include<bits/stdc++.h>
#pragma GCC optimize("Ofast")
#define pb push_back
#define sz(x) (int)(x.size())
#define all(x) x.begin(), x.end()
#define int long long
#define pii pair<int, int>
#define inf 1e9
#define mod 1000000007
#define F first
#define S second
#define wopen(x) freopen((x),"w",stdout)
#define ropen(x) freopen((x),"r",stdin)
#define de(x) cout << #x << " = " << x << ' '
#define IO ios_base::sync_with_stdio(0); cin.tie(0);
using namespace std;
const int mxN = 3e4 + 5;
int n, m, b[mxN], p[mxN];
bitset<mxN * mxN> vis;
bitset<mxN> used;
vector<int> v[mxN];
namespace io {
const int SIZE = 1e7 + 10;
char inbuff[SIZE];
char *l, *r;
inline void init() {
l = inbuff;
r = inbuff + fread(inbuff, 1, SIZE, stdin);
}
inline char gc() {
if (l == r) init();
return (l != r) ? *(l++) : EOF;
}
void read(int &x) {
x = 0; char ch = gc();
while(!isdigit(ch)) ch = gc();
while(isdigit(ch)) x = x * 10 + ch - '0', ch = gc();
}
} using io::read;
signed main() {
IO;
cin >> n >> m;
for(int i = 0; i < m; i++) cin >> b[i] >> p[i], v[b[i]].pb(p[i]);
int ok = 0;
for(int i = 0; i < m; i++) {
if(i == 1) continue;
int t = abs(b[i] - 1);
if(t % p[i] == 0) {
ok = 1;
break;
}
}
if(!ok) {
cout << -1 << '\n';
return 0;
}
queue<array<int, 3>> q;
q.push({b[0], p[0], 0});
while(!q.empty()) {
auto now = q.front(); q.pop();
vis[now[0] * n + now[1]] = 1;
if(now[0] == b[1]) {
cout << now[2] << '\n';
return 0;
}
if(!used[now[0]]) {
for(int i : v[now[0]]) {
if(now[0] + i < n and vis[(now[0] + i) * n + i] == 0)
q.push({now[0] + i, i, now[2] + 1});
if(now[0] - i >= 0 and vis[(now[0] - i) * n + i] == 0)
q.push({now[0] - i, i, now[2] + 1});
}
}
used[now[0]] = 1;
if(now[0] + now[1] < n and vis[(now[0] + now[1]) * n + now[1]] == 0)
q.push({now[0] + now[1], now[1], now[2] + 1});
if(now[0] - now[1] >= 0 and vis[(now[0] - now[1]) * n + now[1]] == 0)
q.push({now[0] - now[1], now[1], now[2] + 1});
}
cout << -1 << '\n';
}
