#include<bits/stdc++.h>
using namespace std;
#define int long long
#define endl '\n'
inline int read(){
	int x=0,k=1;char c=getchar();
	while(c<'0'||c>'9'){
		if(c=='-')k=-1;
		c=getchar();
	}
	while(c>='0'&&c<='9')x=(x<<3)+(x<<1)+(c^48),c=getchar();
	return x*k;
}
const int N=1e6+5;
const double eps=1e-13;
double p[N];
int c[N];
int n;
double cal(double x){
	int sx=0;
	for(int i=1;i<=n;++i)if(p[i]*x>=1){
		sx+=c[i];
	}
	double l=1,r=2;
	int sy=0;
	double m=sx*x;
	while(l+eps<r){
		int cy=0;
		double y=(l+r)/2;
		for(int i=1;i<=n;++i){
			if(p[i]*x>=1)continue;
			if((1-p[i])*y>=1)cy+=c[i];
		}
		if(cy*y-0.0001<m)l=y,sy=max(sy,cy);
		else r=y-eps;
	}
	return sx+sy-sx*x;
}
void sol(){
	n=read();
	for(int i=1;i<=n;++i)cin>>p[i]>>c[i];
	double l=1,r=2;
	while(l+eps<r){
		double lm=l+(r-l)/3,rm=r-(r-l)/3;
//		cout<<lm<<' '<<rm<<endl;
//		cout<<cal(lm)<<"?"<<cal(rm)<<endl;
		if(cal(lm)<cal(rm))l=lm;
		else r=rm;
	}
	printf("%.10f\n",cal((l+r)/2));
}
signed main(){
	int T=1;
	while(T--)sol(); 
}