#include<bits/stdc++.h>
#define db double
using namespace std;
const int N=1e5+10;
db H;
int n;
struct node{int t,h;}a[N];
bool cmp(node x,node y){return x.t<y.t;}
priority_queue<db> q;
int flag;
int main(){
scanf("%lf%d",&H,&n);
for(int i=1;i<=n;i++){
scanf("%d%d",&a[i].t,&a[i].h);
if(!a[i].h) flag=1;
}
if(!flag){puts("-1");return 0;}
sort(a+1,a+n+1,cmp);
db ans=0;
for(int i=1;i<=n;i++){
db t=a[i].t-a[i-1].t;
while(q.size()&&q.top()>=H) q.pop();
while(q.size()&&H-q.top()<=t*1.0*q.size()){
t-=(H-q.top())*1.0/q.size();
H=q.top();
if(H==0){
ans=a[i].t-t;
break;
}
q.pop();
}
if(t>0) H-=t*q.size();
q.push(a[i].h);
}
if(ans){
printf("%.6lf\n",ans);
return 0;
}
ans=a[n].t;
while(q.size()){
ans+=(H-q.top())*1.0/q.size();
H=q.top();
q.pop();
}
printf("%.6lf",ans);
return 0;
}