#include<bits/stdc++.h>
using namespace std;

const int N=1e6+100;
typedef long double db;
const db eps=1e-9;
struct ques{
    db ti;int ty,x;
}p[2*N];db dis[N];
int X[N],Y[N];

bool cmp(ques n1,ques n2) {
    if(n1.ti==n2.ti) return (n1.ty<n2.ty);
    return (n1.ti+eps<n2.ti);
}

db doit(db A,db B,db C,db X){
    return sqrt(A*X*X+B*X+C);
}

int main()
{
    db s;scanf("%Lf",&s);
    int n;scanf("%d",&n);

    X[0]=Y[0]=0;
    for(int i=1;i<=n;i++) scanf("%d%d",&X[i],&Y[i]);

    dis[0]=0;
    for(int i=1;i<=n;i++){
        dis[i]=dis[i-1]+sqrt((X[i]-X[i-1])*1.0*(X[i]-X[i-1])+(Y[i]-Y[i-1])*1.0*(Y[i]-Y[i-1]));
    }
    int tot=0;
    for(int i=1;i<=n;i++){
        p[++tot].ty=0;p[tot].ti=dis[i];p[tot].x=i;
        p[++tot].ty=1,p[tot].ti=dis[i]-s,p[tot].x=i;
    }
    sort(p+1,p+tot+1,cmp);

    db t1,t2,t3,t4,t5,t6,t7,t8,Ans=1e9;
    for(int i=1;i<=tot;i++){
        if(p[i].x==n) break;
        if(p[i].ty==0){
            t1=(X[p[i].x+1]-X[p[i].x])/(dis[p[i].x+1]-dis[p[i].x]);
            t2=X[p[i].x]-t1*p[i].ti;
            t3=(Y[p[i].x+1]-Y[p[i].x])/(dis[p[i].x+1]-dis[p[i].x]);
            t4=Y[p[i].x]-t3*p[i].ti;
        }
        else{
            t5=(X[p[i].x+1]-X[p[i].x])/(dis[p[i].x+1]-dis[p[i].x]);
            t6=X[p[i].x]-t5*p[i].ti;
            t7=(Y[p[i].x+1]-Y[p[i].x])/(dis[p[i].x+1]-dis[p[i].x]);
            t8=Y[p[i].x]-t7*p[i].ti;
        }
        assert((dis[p[i].x+1]-dis[p[i].x])>1e-6);
        if(p[i].ti<-eps) continue;
        db tmp1=t5-t1,tmp2=t6-t2,tmp3=t7-t3,tmp4=t8-t4;
        db A=tmp1*tmp1+tmp3*tmp3,C=tmp2*tmp2+tmp4*tmp4,B=2*(tmp1*tmp2+tmp3*tmp4);
        db Mid=-B/(2*A);
        if(fabs(A)<eps){
            if(B<-eps) Mid=1e18;
            else Mid=-1e18;
        }
        if(Mid>=p[i].ti&&Mid<=p[i+1].ti) Ans=min(Ans,doit(A,B,C,Mid));
        else if(Mid<p[i].ti) Ans=min(Ans,doit(A,B,C,p[i].ti));
        else Ans=min(Ans,doit(A,B,C,p[i+1].ti));
    }
    printf("%.9Lf\n",Ans);

    return 0;
}