#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for (int i=a; i<int(b); i++)
#define all(x) x.begin(),x.end()
int main() {
int n,m;
cin >> n >> m;
vector<pair<int,int> > doges(m);
vector<set<int> > scrapers(n); //For each scraper a set of all dogs that reside there from beginning, or rather their jumps
rep(i,0,m) {
int b,p;
cin >> b >> p;
doges[i]=make_pair(b,abs(p));
scrapers[b].insert(-p);
}
queue<vector<int> > order; //Each element is <numjumps,position, jumplen, 1 or 0 for left/right>
int dp[n];
rep (i,0,n) dp[i]=1000000000;
int start,stop;
start=doges[0].first;
stop=doges[1].first;
dp[start]=0;
for (auto p:scrapers[start]) {
order.push({0,start,-p,1});
order.push({0,start,-p,0});
}
while (!order.empty()) {
int num,pos,p;
num=-order.front()[0];
pos=order.front()[1];
p=order.front()[2];
bool left = order.front()[3]==1;
order.pop();
int next;
if (left) next=pos+p;
else next=pos-p;
if (next>=0 && next<n) {
if (next==stop) {dp[next]=min(num+1,dp[next]); break;}
if (dp[next]>num+1) {
dp[next]=num+1;
for (auto pn:scrapers[next]){
if (-pn!=p) {
order.push({-num-1,next,-pn,1});
order.push({-num-1,next,-pn,0});
}
}
}
order.push({-num-1,next,p,left});
}
}
if (dp[stop] == 1000000000) cout << -1 << endl;
else cout << dp[stop]<< endl;
}
