#include <bits/stdc++.h>
using namespace std;
int num_buildings, num_dogs, target;
int dis[30000];
unordered_set<int> buildings[30000];
int main(int argc, const char * argv[]) {
int start = 0;
cin>>num_buildings>>num_dogs;
for(int i=0; i<num_dogs; i++){
int b, p; cin>>b>>p;
buildings[b].insert(p);
if(i == 0)
start = b;
if(i == 1)
target = b;
}
memset(dis,0x3f,sizeof dis); dis[start] = 0;
priority_queue<pair<int,int>,vector<pair<int,int>>,greater<pair<int,int>>> q;
q.push({0,start});
while(!q.empty()){
int current_building = q.top().second; q.pop();
if(current_building == target)
continue;
for(int check:buildings[current_building]){
for(int i=1; i*check+current_building<num_buildings; i++){
int next_building = i*check+current_building;
if(dis[next_building] > dis[current_building]+i){
dis[next_building] = dis[current_building]+i;
q.push({dis[next_building],next_building});
if(binary_search(buildings[next_building].begin(),buildings[next_building].end(),check))
break;
}
}
for(int i=1; current_building-i*check>=0; i++){
int next_building = current_building-i*check;
if(dis[next_building] > dis[current_building]+i){
dis[next_building] = dis[current_building]+i;
q.push({dis[next_building],next_building});
if(binary_search(buildings[next_building].begin(),buildings[next_building].end(),check))
break;
}
}
}
}
cout<<(dis[target]==0x3f3f3f3f?-1:dis[target]);
return 0;
}
