#include <bits/stdc++.h>
using namespace std;
const int SQRT = 1.7e2;
struct BfsElem{
int node, dir, dist;
};
int solve(int n, int m, vector<vector<int>> doges, int sourceBuilding, int targetBuilding){
vector<vector<bool>> visited(n, vector<bool>(SQRT, false));
queue<BfsElem> bfs;
bfs.push({sourceBuilding, 0, 0});
int ans = -1;
int mults[2] = {1, -1};
while(bfs.size()){
BfsElem elem = bfs.front();
bfs.pop();
if(elem.node < 0 || n <= elem.node) continue;
if(elem.node == targetBuilding){
ans = elem.dist;
break;
}
if(abs(elem.dir) < SQRT){
if(visited[elem.node][abs(elem.dir)]) continue;
visited[elem.node][abs(elem.dir)] = true;
}
while(doges[elem.node].size()){
for(int mult : mults){
int newDir = mult * doges[elem.node].back();
bfs.push({elem.node + newDir, newDir, elem.dist + 1});
}
doges[elem.node].pop_back();
}
if(elem.dir != 0){
bfs.push({elem.node + elem.dir, elem.dir, elem.dist + 1});
}
}
return ans;
}
int solve2(int n, int m, vector<vector<int>> doges, int sourceBuilding, int targetBuilding){
// vector<vector<bool>> visited(n, vector<bool>(n, false));
// auto comp = [](const BfsElem& left, const BfsElem& right){
// left.dist > right.dist;
// };
// priority_queue<BfsElem, vector<BfsElem>, decltype(comp)> bfs(comp);
queue<BfsElem> bfs;
bfs.push({sourceBuilding, 0, 0});
int ans = -1;
int mults[2] = {1, -1};
while(bfs.size()){
BfsElem elem = bfs.front();
bfs.pop();
if(elem.node < 0 || n <= elem.node) continue;
if(elem.node == targetBuilding){
ans = elem.dist;
break;
}
// if(abs(elem.dir) < visited[elem.node].size()){
// if(visited[elem.node][abs(elem.dir)]) continue;
// visited[elem.node][abs(elem.dir)] = true;
// }
while(doges[elem.node].size()){
for(int mult : mults){
int newDir = mult * doges[elem.node].back();
bfs.push({elem.node + newDir, newDir, elem.dist + 1});
}
doges[elem.node].pop_back();
}
if(elem.dir == 0) continue;
bfs.push({elem.node + elem.dir, elem.dir, elem.dist + 1});
}
return ans;
}
int main(){
cin.tie(nullptr);
ios_base::sync_with_stdio(false);
int n, m;
cin>>n>>m;
vector<vector<int>> doges(n);
int sourceBuilding;
int targetBuilding;
for(int i=0; i<m; i++){
int b, p;
cin>>b>>p;
if(i == 0){
sourceBuilding = b;
}
if(i == 1){
targetBuilding = b;
}
doges[b].push_back(p);
}
int ans = solve(n, m, doges, sourceBuilding, targetBuilding);
cout<<ans;
}
