#include <bits/stdc++.h>
#define FIXED_FLOAT(x) std::fixed <<std::setprecision(20) << (x)
#define all(v) (v).begin(), (v).end()
using namespace std;
#define forn(i,n) for (int i = 0; i < (n); ++i)
#define rforn(i, n) for(int i = (n) - 1;i >= 0;--i)
using ll = long long;
int mod = (ll)1e9 + 7;
#define PI acos(-1)
typedef pair<int, int> pairs;
const int INF = 1e9 + 1;
const int N = 2e5 + 100;
const double eps = 1e-7;
template <class T> using V = vector<T>;
template <class T> using VV = V<V<T>>;
template <typename XPAX>
void ckma(XPAX &x, XPAX y) {
x = (x < y ? y : x);
}
template <typename XPAX>
void ckmi(XPAX &x, XPAX y) {
x = (x > y ? y : x);
}
void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
void solve() {
int n, m;
cin >> n >> m;
V<set<int>> s(n);
VV<pairs> g(n);
V<int> f(m), p(m);
V<int> vis(n, false);
forn(i, m) {
cin >> f[i] >> p[i];
s[f[i]].insert(p[i]);
}
int x;
for(int i = 0;i < n;++i) {
for(auto c : s[i]) {
x = i - c;
while(x >= 0) {
if(s[x].size())g[i].push_back({(i - x) / c, x});
if(s[x].count(c))break;
x -= c;
}
x = i + c;
while(x < n) {
if(s[x].size())g[i].push_back({(x - i) / c, x});
if(s[x].count(c))break;
x += c;
}
}
}
priority_queue<pairs, V<pairs>, greater<pairs>> q;
q.push({0, f[0]});
//debug(dis);
while(q.size()) {
x = q.top().second;
int s = q.top().first;
q.pop();
if(vis[x])continue;
vis[x] = 1;
if(x == f[1]) {
cout << s << '\n';
exit(0);
}
for(auto c : g[x]) {
if(!vis[c.second])q.push({s + c.first, c.second});
}
}
cout << -1 << '\n';
}
void test_case() {
int t;
cin >> t;
forn(p, t) {
//cout << "Case #" << p + 1 << ": ";
solve();
}
}
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
solve();
}
