#include <bits/stdc++.h>
using namespace std;
#define ll long long
#pragma GCC optimize("O3")
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#define usecppio ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define all(x) ((x).begin()),((x).end())
using pii = pair <int, int>;
#define INF 0x7f7f7f7f
const bool debug = false;
const int MX = 30202;
struct Edge
{
int dest, w;
bool operator<(const Edge &p) const
{
return w > p.w;
}
};
bool relax(Edge edge, int u, int dist[])
{
bool flag = 0;
int v = edge.dest, w = edge.w;
if (dist[v] > dist[u] + w && (dist[u]!=INF))
{
flag = true;
dist[v] = dist[u]+w;
}
return flag;
}
void dijkstra(int dist[], int start, vector<Edge> graph[])
{
fill(dist,dist+MX,INF);
dist[start] = 0;
priority_queue<Edge> pq;
pq.push({start,0});
while(!pq.empty())
{
Edge x = pq.top();
int v = x.dest, w = x.w;
pq.pop();
if (w>dist[v])
continue;
for (auto ed : graph[v])
if (relax(ed, v, dist))
pq.push({ed.dest,dist[ed.dest]});
}
}
struct doge
{
int start, power, index;
};
int n, m;
vector <int> bi(30303, 0), pi(30303, 0);
vector <Edge> G[30303];
bitset <30303> mark[30303];
vector <doge> alldoge[30303];
vector <doge> dogeinfo;
int distances[30303];
bool visit[30303];
bool dogevisit[30303];
void print_graph()
{
for (int i = 0; i<n; i++)
{
for (auto it:G[i])
printf("From %d to %d, %d\n",i, it.dest, it.w);
}
}
void dfs(doge curdoge)
{
dogevisit[curdoge.index] = true;
for (auto it:alldoge[curdoge.start])
if (!dogevisit[it.index])
dfs(it);
for (int i = 1; ;i++)
{
int u = curdoge.start+i*curdoge.power;
if (u >= n)
break;
bool check = false;
if (!visit[u])
{
visit[u] = true;
for (auto it:alldoge[u])
{
if (!dogevisit[it.index])
{
if (it.power == curdoge.power)
check = true;
dfs(it);
}
}
}
G[curdoge.start].push_back({u,i});
if (check)
break;
}
for (int i = -1; ;i--)
{
int u = curdoge.start+i*curdoge.power;
if (u < 0)
break;
bool check = false;
if (!visit[u])
{
visit[u] = true;
for (auto it:alldoge[u])
{
if (!dogevisit[it.index])
{
if (it.power == curdoge.power)
check = true;
dfs(it);
}
}
}
G[curdoge.start].push_back({u,-i});
if (check) break;
}
}
int32_t main()
{
usecppio
cin >> n >> m;
for (int i = 0; i<m; i++)
{
cin >> bi[i] >> pi[i];
if (!mark[bi[i]][pi[i]] && i!=1)
{
mark[bi[i]][pi[i]] = true;
alldoge[bi[i]].push_back({bi[i], pi[i], i});
}
dogeinfo.push_back({bi[i], pi[i], i});
}
dogevisit[0] = true;
visit[dogeinfo[0].start] = 1;
dfs(dogeinfo[0]);
//print_graph();
dijkstra(distances,bi[0],G);
if (distances[bi[1]] >= 10000000)
{
printf("-1");
}
else printf("%lld\n",distances[bi[1]]);
}
