#include<bits/stdc++.h>
using namespace std;
#define FOR(i, a, b) for(int i = a; i < b; ++i)
#define REP(i, n) FOR(i, 0, n)
#define _ << " " <<
#define sz(x) ((int) x.size())
#define pb(x) push_back(x)
#define TRACE(x) cerr << #x << " = " << x << endl
typedef long long ll;
typedef pair<int, int> point;
const int mod = 1e9 + 7;
int add(int x, int y) {x += y; if(x >= mod) return x - mod; return x;}
int sub(int x, int y) {x -= y; if(x < 0) return x + mod; return x;}
int mul(int x, int y) {return (ll) x * y % mod;}
const int MAXN = 3e4 + 5;
int n, m, dist[MAXN], start[MAXN];
set<int> S[MAXN];
vector <point> E[MAXN];
void update(int pos, int range){
int cnt = 1;
for(int i = pos + range; i < n; i += range){
E[pos].pb(point(i, cnt));
cnt ++;
if(S[i].find(range) != S[i].end()) break;
}
cnt = 1;
for(int i = pos - range; i >= 0; i -= range){
E[pos].pb(point(i, cnt));
cnt ++;
if(S[i].find(range) != S[i].end()) break;
}
}
const int inf = 1e9;
int dijkstra(){
priority_queue <point> Q;
Q.push(point(0, start[0]));
vector <int> d(MAXN, inf);
d[start[0]] = 0;
while(!Q.empty()){
auto tmp = Q.top(); Q.pop();
int distance = -tmp.first;
int cvor = tmp.second;
d[cvor] = distance;
if(cvor == start[1]) break;
for(auto e : E[cvor]){
int ncvor = e.first;
int ndist = distance + e.second;
if(ndist < d[ncvor]){
d[ncvor] = ndist;
Q.push(point(-ndist, ncvor));
}
}
}
if(d[start[1]] == inf) return -1;
else return d[start[1]];
}
int main(){
ios_base::sync_with_stdio(false); cin.tie(0);
cin >> n >> m;
REP(i, m)
cin >> start[i] >> dist[i];
REP(i, m)
if(start[i] != start[1])
S[start[i]].insert(dist[i]);
REP(i, n){
if(i == start[1]) continue;
for(auto it : S[i])
update(i, it);
}
cout << dijkstra();
}
