#include <bits/stdc++.h>
using namespace std;
#define int long long
const int inf = 2e18;
const int mxN = 2005;
vector<int> pos[mxN];
int n, m, x, y, dp[mxN][mxN];
signed main(){
ios::sync_with_stdio(0);
cin.tie(nullptr); cout.tie(nullptr);
cin >> n >> m;
vector<int> d(m), s(m);
for(int i = 0; i < mxN; ++i){
for(int j = 0; j < mxN; ++j){
dp[i][j] = inf;
}
}
for(int i = 0; i < m; ++i){
cin >> d[i] >> s[i];
if(i == 0) x = d[i], y = s[i];
pos[d[i]].push_back(s[i]);
}
priority_queue<tuple<int, int, int>> pq;
pq.push({0, x, y});
dp[x][y] = 0;
int ans = inf;
while(pq.size()){
auto [dist, id, mov] = pq.top();
pq.pop();
dist = -dist;
if(id == 1){
ans = min(ans, dist);
continue;
}
//if(dp[id][mov] < dist) continue;
for(auto it : pos[id]){
if((id + it) <= (n - 1) && dp[id + it][it] > (dist + 1)){
dp[id + it][it] = dist + 1;
pq.push({-dp[id + it][it], id + it, it});
}
if((id - it) >= 0 && dp[id - it][it] > (dist + 1)){
dp[id - it][it] = dist + 1;
pq.push({-dp[id - it][it], id - it, it});
}
}
if((id + mov) <= (n - 1) && dp[id + mov][mov] > (dist + 1)){
dp[id + mov][mov] = dist + 1;
pq.push({-dp[id + mov][mov], id + mov, mov});
}
if((id - mov) >= 0 && dp[id - mov][mov] > (dist + 1)){
dp[id - mov][mov] = dist + 1;
pq.push({-dp[id - mov][mov], id - mov, mov});
}
}
if(ans == inf) cout << -1 << "\n";
else cout << ans << "\n";
return 0;
}
