#include <cstdio>
#include <queue>
#include <vector>
#include <cstring>
#include <algorithm>
using namespace std;
struct data{
int to,dist;
};
struct cmp{
bool operator()(data d1,data d2){
return d1.dist>d2.dist;
}
};
int n,m;
int dist[2010];
int graph[2010][2010];
data arr[30010];
bool visit[2010];
void dij(int a){
int i,j,flag=1,now;
data s;
while(flag){
flag=0; now=21e8;
for(i=0;i<n;i++){
if(!visit[i] && dist[i] && dist[i]<now) a=i,flag=1,now=dist[i];
}
if(!flag) break; visit[a]=1;
for(i=0;i<n;i++){
if(!graph[a][i]) continue;
if(!dist[i] || dist[i]>dist[a]+graph[a][i]) dist[i]=dist[a]+graph[a][i];
}
}
}
int main(){
int i,j;
scanf("%d %d",&n,&m);
for(i=0;i<m;i++) scanf("%d %d",&arr[i].to,&arr[i].dist),visit[arr[i].to]=1;
for(i=0;i<m;i++){
for(j=0;j<n;j++){
if(!(abs(arr[i].to-j)%arr[i].dist)){
if(!graph[arr[i].to][j]) graph[arr[i].to][j]=abs(arr[i].to-j)/arr[i].dist;
else graph[arr[i].to][j]=min(graph[arr[i].to][j],abs(arr[i].to-j)/arr[i].dist);
}
}
}
memset(visit,0,sizeof(visit));
dist[arr[0].to]=1;
dij(0);
if(!visit[arr[1].to]) printf("-1");
else printf("%d",dist[arr[1].to]-1);
return 0;
}
