#include <bits/stdc++.h>
using namespace std;
typedef pair<int,int> pii;
typedef pair<int,pii> pip;
#define f first
#define s second
const int maxn = 3e4+10, root = 180, inf = 0x3f3f3f3f;
int b[maxn], p[maxn], dist1[maxn], dist2[maxn][root];
vector<int> doges[maxn];
int main(){
ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
int n, m;
cin >> n >> m;
for(int i=0; i<m; i++){
cin >> b[i] >> p[i];
doges[b[i]].push_back(p[i]);
}
queue<pip> fila;
memset(dist1, inf, sizeof dist1);
memset(dist2, inf, sizeof dist2);
fila.push({0, {b[0], p[0]}});
while(!fila.empty()){
int d = fila.front().f, u = fila.front().s.f, power = fila.front().s.s;
fila.pop();
if(power <= root){
if(dist2[u][power] != inf) continue;
dist2[u][power] = d;
if(u + power < n) fila.push({d + 1, {u + power, power}});
if(u - power >= 0) fila.push({d + 1, {u - power, power}});
for(auto x : doges[u]){
if(u + x < n) fila.push({d + 1, {u + x, x}});
if(u - x >= 0) fila.push({d + 1, {u - x, x}});
}
}
else{
if(dist1[u] != inf) continue;
dist1[u] = d;
if(u + power < n) fila.push({d + 1, {u + power, power}});
if(u - power >= 0) fila.push({d + 1, {u - power, power}});
for(auto x : doges[u]){
if(u + x < n) fila.push({d + 1, {u + x, x}});
if(u - x >= 0) fila.push({d + 1, {u - x, x}});
}
}
}
int ans = dist1[b[1]];
for(int i=1; i<=root; i++) ans = min(ans, dist2[b[1]][i]);
cout << (ans == inf ? -1 : ans) << "\n";
return 0;
}
