#include <bits/stdc++.h>
using namespace std;
const int nx=2e3+5;
int n, m, st, ed, jp, b, p[nx], dp[nx][nx];
vector<int> v[nx];
queue<tuple<int, int, int>> q;
int main()
{
cin.tie(NULL)->sync_with_stdio(false);
cin>>n>>m>>st>>p[0]>>ed>>p[1]; v[ed].push_back(1);
for (int i=1; i<m-1; i++) cin>>b>>p[i+1], v[b].push_back(i+1);
for (int i=0; i<nx; i++) for (int j=0; j<nx; j++) dp[i][j]=INT_MAX;
dp[0][st]=0;
q.push({0, 0, st});
while (!q.empty())
{
auto [c, w, l]=q.front();
if (c==1)
{
cout<<dp[1][ed];
return 0;
}
//cout<<c<<' '<<w<<' '<<l<<'\n';
q.pop();
if (l+p[c]<n&&w+1<dp[c][l+p[c]]) dp[c][l+p[c]]=w+1, q.push({c, w+1, l+p[c]});
if (l-p[c]>=0&&w+1<dp[c][l-p[c]]) dp[c][l-p[c]]=w+1, q.push({c, w+1, l-p[c]});
for (auto nw:v[l]) if (w<dp[nw][l]) dp[nw][l]=w, q.push({nw, w, l});
}
}
