#include<bits/stdc++.h> // #include <ext/pb_ds/assoc_container.hpp> // #include <ext/pb_ds/tree_policy.hpp> using namespace std; // #define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update> // using namespace __gnu_pbds; typedef long long ll; #define int long long #define vi vector<int> #define vvi vector<vi> #define vll vector<ll> #define vvll vector<vll> #define pi pair<int,int> #define pll pair<ll,ll> #define ff first #define ss second #define vpi vector<pair<int,int>> #define rep(ii,st, n) for(int ii=st; ii<n; ii++) #define gp " " //bit_manupulation #define checkbit(x,n) (x&(1LL<<n)) #define setbit(x,n) (x=(x|(1LL<<n))) #define resetbit(x,n) (x=(x&(~(1LL<<n)))) #define pow2(i) (1LL<<i) #define bitcnt(x) ((sizeof(x) <= sizeof(int)) ? (32 - __builtin_clz(x)) : (64 - __builtin_clzll(x))) // #define DEBG #define debug(n) #define debugc(a) #define debugcc(a) #ifdef DEBG #define debug(n) cout<<__LINE__<<gp<<#n<<gp<<n<<endl; #define debugc(a) cout<<__LINE__<<gp<<#a<<gp<<'['<<gp;for(auto el:a){cout<<el<<gp;}cout<<']'<<endl; #define debugcc(a) cout<<__LINE__<<gp<<#a<<gp<<'['<<gp;for(auto el:a){cout<<'{'<<gp<<el.ff<<','<<el.ss<<gp<<'}'<<gp;}cout<<']'<<endl; #endif #define fastcin() ios_base::sync_with_stdio(false); cin.tie(NULL); #define endl '\n' #define All(a) a.begin(),a.end() template<typename T> void get_vector(T&a){for(auto&e:a)cin>>e;} template<typename T> void put_vector(T a){for(auto e:a)cout<<e<<" ";cout<<endl;} const ll INF = 2e18; const ll inf = INT_MAX; const ll M = 2007; const ll N = 107; const ll modinvof2 = 500000004; //==============================CODE STARTS HERE==============================// int n,m; vi b(M),p(M); map<int,vi>mp; // vector<vector<int>>visited(N, vi(M)); int dpp[N][M][3]; int func(int i, int who, int si){ if(i<0 || i>=n) return 1e16; if(i == b[1]){ return 0; } if(dpp[i][who][si] != -1) return dpp[i][who][si]; int ans = 1e16; if(si != 2) ans = min(ans,func(i+p[who], who, 1)+1); if(si != 1) ans = min(ans,func(i-p[who], who , 2)+1); for(auto& el: mp[i]){ ans = min(ans,func(i+p[el], el, 1)+1); ans = min(ans,func(i-p[el], el, 2)+1); } return dpp[i][who][si] = ans; } void preprocessing(){ } void solve(int testcases){ cin>>n>>m; rep(i,0,n){ rep(j,0,m){ rep(k,0,3){ dpp[i][j][k] = -1; } } } for(int i = 0; i<m; i++){ cin>>b[i]>>p[i]; mp[b[i]].push_back(i); } for(auto& el: mp){ debug(el.first) debugc(el.second) } int mx = 0; bool fl = false; int ans = func(0,0, 0); if(ans >= 1e13){ cout<<-1<<endl; return; } cout<<ans<<endl; } int32_t main(){ fastcin(); int t=1; // cin>>t; preprocessing(); rep(i,1,t+1){ solve(i); } return 0; }