#include<bits/stdc++.h>
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx,avx2,bmi,bmi2,popcnt,lzcnt")
using namespace std;
void solve(){
short n, m;
cin >> n >> m;
short b[m], p;
vector<short> h[n], sm[n];
const short s = 300;
for(int i = 0;i < m;i++){
cin >> b[i] >> p;
if(p>=s) h[b[i]].push_back(p);
else sm[b[i]].push_back(p);
}
int d[n][s];
bitset<s> v[n];
for(int i = 0;i < n;i++) for(int j = 0;j < s;j++) d[i][j] = INT_MAX, v[i][j] = false;
d[b[0]][0] = 0;
priority_queue<pair<int, pair<short, short>>,vector<pair<int, pair<short, short>>>, greater<pair<int, pair<short, short>>>> q;
q.push({0, {b[0], 0}});
while(!q.empty()){
short wi = q.top().second.first;
short po = q.top().second.second;
if(v[wi][po]) {
q.pop();
continue;
}
if(wi == b[1]){
cout << d[wi][po] << '\n';
return;
}
v[wi][po] = true;
q.pop();
if(po){
if(wi+po < n && d[wi+po][po] > 1+d[wi][po]){
d[wi+po][po] = 1+d[wi][po];
q.push({d[wi+po][po], {wi+po, po}});
if(1+d[wi][po] < d[wi+po][0]){
d[wi+po][0] = 1+d[wi][po];
q.push({d[wi+po][0], {wi+po, 0}});
}
}
if(wi-po >= 0 && d[wi-po][po] > 1+d[wi][po]){
d[wi-po][po] = 1+d[wi][po];
q.push({d[wi-po][po], {wi-po, po}});
if(1+d[wi][po] < d[wi-po][0]){
d[wi-po][0] = 1+d[wi][po];
q.push({d[wi-po][0], {wi-po, 0}});
}
}
continue;
}
#pragma GCC ivdep
for(int u : h[wi]){
for(int j = wi+u,c=1;j < n;j += u,c++) {
if(d[j][0] > d[wi][0]+c){
d[j][0] = d[wi][0]+c;
q.push({d[j][0], {j, 0}});
}
}
for(int j = wi-u,c=1;j >= 0;j -= u,c++) {
if(d[j][0] > d[wi][0]+c){
d[j][0] = d[wi][0]+c;
q.push({d[j][0], {j, 0}});
}
}
}
#pragma GCC ivdep
for(int po : sm[wi]){
if(v[wi][po]) continue;
d[wi][po] = d[wi][0];
if(wi+po < n && d[wi+po][po] > 1+d[wi][po]){
d[wi+po][po] = 1+d[wi][po];
q.push({d[wi+po][po], {wi+po, po}});
if(1+d[wi][po] < d[wi+po][0]){
d[wi+po][0] = 1+d[wi][po];
q.push({d[wi+po][0], {wi+po, 0}});
}
}
if(wi-po >= 0 && d[wi-po][po] > 1+d[wi][po]){
d[wi-po][po] = 1+d[wi][po];
q.push({d[wi-po][po], {wi-po, po}});
if(1+d[wi][po] < d[wi-po][0]){
d[wi-po][0] = 1+d[wi][po];
q.push({d[wi-po][0], {wi-po, 0}});
}
}
}
}
cout << "-1\n";
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
solve();
}
