#include <bits/stdc++.h>
using namespace std;
inline long long msb(long long val){return 63-__builtin_clzll(val);}
inline int msb(int val){return 31-__builtin_clz(val);}
//~ #define int long long
#define FASTIO ios_base::sync_with_stdio(false); cin.tie(NULL);
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define pii pair<int, int>
#define ff first
#define ss second
#define PI acos(-1)
#define ld long double
template<class T> bool chmin(T& a, const T& b) {return a>b? a=b, true:false;}
template<class T> bool chmax(T& a, const T& b) {return a<b? a=b, true:false;}
const int mod = 1e9+7, N = 30005, M = 30005;
int n, m, k;
bitset<M> used[N];
int len[N];
vector<int> who_is_here[N];
void solve(int test_case){
int i, j;
cin >> n >> m;
int start;
for(i=0;i<m;i++){
int pos;
cin >> pos >> len[i];
if(i){
who_is_here[pos].pb(i);
}else {
start = pos;
}
}
priority_queue<tuple<int,int,int>,vector<tuple<int,int,int>>, greater<tuple<int,int,int>>> q;
q.push({0,start,0});
used[start][0] = 1;
while(!q.empty()){
auto [dist, pos, i] = q.top(); q.pop();
if(i == 1){
cout << dist << '\n';
return;
}
while(who_is_here[pos].size()){
auto x = who_is_here[pos].back();
used[pos][x] = 1;
q.push({dist,pos,x});
who_is_here[pos].pop_back();
}
if(pos - len[i] >= 0 && !used[pos-len[i]][i]){
used[pos-len[i]][i] = 1;
q.push({dist+1,pos-len[i],i});
}
if(pos + len[i] < n && !used[pos+len[i]][i]){
used[pos+len[i]][i] = 1;
q.push({dist+1,pos+len[i],i});
}
}
cout << -1 << '\n';
return;
}
signed main(){
FASTIO;
//~ #define MULTITEST 1
#if MULTITEST
int _T;
cin >> _T;
for(int T_CASE = 1; T_CASE <= _T; T_CASE++)
solve(T_CASE);
#else
solve(1);
#endif
return 0;
}
