#include<bits/stdc++.h>
using namespace std;
vector<pair<int,int> >v[300005];
map<pair<int,int>, int> sto;
priority_queue<pair<int,int>,vector<pair<int,int> >, greater<pair<int,int> > >pq;
main(){
cin.tie(0);
ios::sync_with_stdio(0);
int N,M;
cin>>N>>M;
bool occ[N+3];
memset(occ,0,sizeof(occ));
vector<pair<int,int> >hi;
for(int i=0;i<M;i++){
int a,b;
cin>>a>>b;
occ[a] = 1;
hi.push_back(make_pair(a,b));
}
for(int i=0;i<M;i++){
int x = 1;
while(hi[i].first-x*hi[i].second>=0){
//if(occ[hi[i].first-x*hi[i].second])v[hi[i].first].push_back(make_pair(hi[i].first-x*hi[i].second,x));
int y;
if(sto.find(make_pair(hi[i].first,hi[i].first-x*hi[i].second))!=sto.end()) y = sto[make_pair(hi[i].first,hi[i].first-x*hi[i].second)];
else y = 1e9;
if(occ[hi[i].first-x*hi[i].second]) sto[make_pair(hi[i].first,hi[i].first-x*hi[i].second)] = min(y,x);
//if(occ[hi[i].first-x*hi[i].second])sto[hi[i].first][hi[i].first-x*hi[i].second] = min(sto[hi[i].first][hi[i].first-x*hi[i].second],x);
x++;
}
x = 1;
while(hi[i].first+x*hi[i].second<N){
int y;
if(sto.find(make_pair(hi[i].first,hi[i].first+x*hi[i].second))!=sto.end()) y = sto[make_pair(hi[i].first,hi[i].first+x*hi[i].second)];
else y = 1e9;
//if(occ[hi[i].first+x*hi[i].second])v[hi[i].first].push_back(make_pair(hi[i].first+x*hi[i].second,x));
//if(occ[hi[i].first+x*hi[i].second])sto[hi[i].first][hi[i].first+x*hi[i].second] = min(sto[hi[i].first][hi[i].first+x*hi[i].second],x);
if(occ[hi[i].first+x*hi[i].second]) sto[make_pair(hi[i].first,hi[i].first+x*hi[i].second)] = min(y,x);
x++;
}
}
for(auto it = sto.begin();it!=sto.end();it++){
v[it->first.first].push_back(make_pair(it->first.second,it->second));
}
/*for(int i=0;i<N;i++){
for(int j=0;j<N;j++){
if(sto[i][j]!=1e9) v[i].push_back(make_pair(j,sto[i][j]));
}
}*/
pq.push(make_pair(0,hi[0].first));
int dist[300005];
for(int i=0;i<300004;i++) dist[i] = 1e9;
dist[hi[0].first] = 0;
while(!pq.empty()){
int dis = pq.top().first;
int no = pq.top().second;
pq.pop();
if(dist[no]<dis)continue;
for(pair<int,int>p:v[no]){
if(dis+p.second<dist[p.first]){
dist[p.first] = dis+p.second;
pq.push(make_pair(dis+p.second,p.first));
}
}
}
if(dist[hi[1].first] == 1e9) cout<<"-1";
else cout<<dist[hi[1].first];
}
