#include <bits/stdc++.h>
#define MT make_tuple
using namespace std;
typedef tuple<int,int,int> trinca;
typedef pair<trinca,int> Ttrinca;
const int MAXN = 30010;
const int BUCKET = 175;
vector<trinca> grafo[MAXN];
queue<trinca> filas[BUCKET];
priority_queue<Ttrinca, vector<Ttrinca> , greater<Ttrinca> > Dijkstra;
int processado[MAXN][BUCKET],N,M,origem,destino;
trinca top(){
return Dijkstra.top().first;
}
void pop(){
int peso = Dijkstra.top().second;
Dijkstra.pop();
filas[peso].pop();
if(!filas[peso].empty()) Dijkstra.push(make_pair(filas[peso].front(),peso));
}
void push(int peso,trinca davez){
if(filas[peso].empty()) Dijkstra.push(make_pair(davez,peso));
filas[peso].push(davez);
}
bool empty(){
return Dijkstra.empty();
}
int main(){
scanf("%d %d",&N,&M);
for(int i = 0;i<M;i++){
int B,P;
scanf("%d %d",&B,&P);
if(i == 0) origem = B;
else if(i == 1) destino = B;
if(P < BUCKET){
grafo[B].push_back(MT(B,P,0));
}
else{
for(int j = 1;B - j*P >= 0;j++){
grafo[B].push_back(MT(B - j*P,0,j));
}
for(int j = 1;B + j*P < N;j++){
grafo[B].push_back(MT(B + j*P,0,j));
}
}
}
priority_queue<trinca, vector<trinca>, greater<trinca> > Dijkstra;
push(0,MT(0,origem,0));
while(!empty()){
trinca davez = top();
pop();
int dist = get<0>(davez), v = get<1>(davez), power = get<2>(davez);
if(processado[v][power] == 1) continue;
processado[v][power] = 1;
if(!processado[v][0]){
push(0,MT(dist,v,0));
}
if(power < BUCKET && v - power >= 0){
if(!processado[v - power][power]) push(1,MT(dist+1,v - power,power));
}
if(power < BUCKET && v + power < N){
if(!processado[v + power][power]) push(1,MT(dist+1,v + power,power));
}
if(v == destino){
printf("%d\n",dist);
return 0;
}
if(power != 0) continue;
for(trinca nova : grafo[v]){
int nv = get<0>(nova), npower = get<1>(nova), nadd = get<2>(nova);
if(v == nv){
if(processado[v][npower]) continue;
processado[v][npower] = 1;
if(npower < BUCKET && v - npower >= 0){
if(!processado[v - npower][npower]) push(1,MT(dist+1,v - npower,npower));
}
if(npower < BUCKET && v + npower < N){
if(!processado[v + npower][npower]) push(1,MT(dist+1,v + npower,npower));
}
continue;
}
if(!processado[nv][npower]) push(nadd,MT(dist+nadd,nv,npower));
}
}
printf("-1\n");
return 0;
}
