//
// Created by bg Koh on 2020/10/21.
//
#include <iostream>
#include <vector>
#include <unordered_map>
#include <queue>
using namespace std;
int N, M;
vector<vector<int>> jump;
vector<int> discovered;
vector<unordered_map<int,int>> adjList;
int dfs(int startBuild, int jmp, int time, unordered_map<int, int>& soFar, int dir )
{
if (startBuild >= N || startBuild < 0)
return 0;
if (soFar.find(startBuild) != end(soFar)) {
if (soFar[startBuild] > time)
soFar[startBuild] = time;
} else {
soFar.emplace(startBuild, time);
}
dfs(startBuild+((dir == 1) ? jmp:-jmp), jmp, time+1, soFar, dir);
return 1;
}
void makeGraph()
{
for (int i=0; i<N; ++i) {
for (int j : jump[i]) {
unordered_map<int, int> tmp;
dfs(i+j, j, 1, adjList[i], 1); //right
dfs(i-j, j, 1, adjList[i], 0); //left
#if 0
cout << "\n\n" << i << " building:" << endl;
for (auto& k : adjList[i]) {
cout << k.first << ":" << k.second << endl;
}
#endif
}
}
}
int firstDogeLoc = 0, firstJump = 0;
int secondDogeLoc= 0, secondJump = 0;
void input()
{
cin >> N >> M;
jump = vector<vector<int>>(N,vector<int>());
cin >> firstDogeLoc >> firstJump;
jump[firstDogeLoc].emplace_back(firstJump);
cin >> secondDogeLoc >> secondJump;
jump[secondDogeLoc].emplace_back(secondJump);
for (int i=2; i<M; ++i) {
int start, j;
cin >> start >> j;
jump[start].emplace_back(j);
}
adjList = vector<unordered_map<int, int>>(N, unordered_map<int,int>());
}
vector<int> shortTime;
int dijkstra()
{
shortTime = vector<int>(N, (int)2e9);
priority_queue<pair<int, int>> q;
q.emplace(0, firstDogeLoc);
while (!q.empty()) {
auto [dist, build] = q.top();
dist = -dist;
q.pop();
if (shortTime[build] <= dist)
continue;
shortTime[build] = dist;
//for (const pair<int,int>& adj : adjList[build]) {
for (const auto& adj : adjList[build]) {
int there= adj.first;
int distanceToThere = adj.second + dist;
if (shortTime[there] > distanceToThere) {
shortTime[there] = distanceToThere;
q.emplace(-distanceToThere, there);
}
}
}
return shortTime[secondDogeLoc] != 2e9 ? shortTime[secondDogeLoc] : -1;
}
int main()
{
ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);
input();
makeGraph();
cout << dijkstra() << endl;
#if 0
unordered_map<int,int> test;
//dfs(2, 2, 1, test, 1);
dfs(4, 2, 1, test, 0);
for (auto& i : test) {
cout << i.first << ":" << i.second << endl;
}
#endif
return 0;
}
