#pragma GCC optimize("Ofast")
#pragma GCC optimize ("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#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];
unordered_set<int> S[MAXN];
vector <point> E[MAXN];
struct Edge{
int x, cost;
Edge(){}
Edge(int x, int cost) : x(x), cost(cost) {}
friend bool operator < (const Edge &A, const Edge &B){
return A.cost > B.cost;
}
};
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;
}
}
int dijkstra(){
priority_queue <Edge> Q;
Q.push(Edge(start[0], 0));
vector <int> d(n, -1);
while(!Q.empty()){
while(!Q.empty() && d[Q.top().x] != -1) Q.pop();
if(Q.empty()) break;
auto edge = Q.top();
d[edge.x] = edge.cost;
if(edge.x == start[1]) break;
for(auto it : S[edge.x])
update(edge.x, it);
for(auto e : E[edge.x]){
int ncvor = e.first, ndist = e.second;
if(d[ncvor] != -1) continue;
Q.push(Edge( ncvor, edge.cost + ndist ));
}
}
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]);
cout << dijkstra();
}
