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#include<iostream>
#include<queue>
#include<cmath>
#include<cstdio>
using namespace std;
const long long maxn=1000005;
const double INF=999999999;
#define pp pair<double,int>
#define mp make_pair
int n,m;
struct node
{
int nex,v;
double val;
}edge[maxn];
bool vis[maxn];int head[maxn],cnt;
double dis[maxn];
double px[maxn],py[maxn];
void add(int u,int v,double val)
{
edge[++cnt].nex=head[u];
edge[cnt].val=val;
edge[cnt].v=v;
head[u]=cnt;
}
priority_queue<pp,vector<pp>,greater<pp> > q;
void dij()
{
for(int i=1;i<=m+2;i++)
dis[i]=INF;
dis[m+1]=0;
q.push(mp(0,m+1));
while(!q.empty())
{
int u=q.top().second;
q.pop();
if(vis[u]) continue;
vis[u]=1;
for(int i=head[u];i;i=edge[i].nex)
{
double val=edge[i].val;
int v=edge[i].v;
if(dis[v]>max(dis[u],val))
{
dis[v]=max(dis[u],val);
q.push(mp(dis[v],v));
}
}
}
}
double getdis(int x,int y)
{
return sqrt((px[x]-px[y])*(px[x]-px[y])+(py[x]-py[y])*(py[x]-py[y]));
}
int main()
{
cin>>n>>m;
for(int i=1;i<=m;i++)
{
cin>>px[i]>>py[i];
add(m+1,i,px[i]);
add(i,m+1,px[i]);
add(m+2,i,double(n)-px[i]);
add(i,m+2,double(n)-px[i]);
}
for(int i=1;i<=m;i++)
{
for(int j=i+1;j<=m;j++)
{
add(i,j,getdis(i,j)/2.0);
add(j,i,getdis(i,j)/2.0);
}
}
dij();
printf("%.2lf",dis[m+2]);
}
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