C++ :

#include<cstdio>
#include<cstring>
#include<cstdio>
using namespace std;
#define Size 1010
int n;
double tubarea,tubheight,watersize;
double boxlenth[Size]={0},boxden[Size]={0},boxsize[Size]={0};
bool judge(double);
int main()
{
	scanf("%lf%lf%lf",&tubarea,&tubheight,&watersize);
	scanf("%d",&n);
	for(int i=1;i<=n;i++)
	{
		scanf("%lf%lf",&boxlenth[i],&boxden[i]);
		if(boxden[i]<=1)
		boxsize[i]=boxden[i]*boxlenth[i];
		else
		boxsize[i]=boxlenth[i];
	}
	double left=watersize/tubarea,right=tubheight,middle;
	while(right-left>1e-9)
	{
		middle=(left+right)/2;
		if(judge(middle))
		right=middle;
		else
		left=middle;
	}
	printf("%.7lf",left);
	return 0;
}
bool judge(double waterheight)
{
	double devote=0;
	for(int i=1;i<=n;i++)
	{
		if(boxsize[i]>waterheight)
		devote=devote+waterheight*boxlenth[i]*boxlenth[i];
		else
		{
			if(waterheight+boxlenth[i]-boxsize[i]>tubheight)
			devote=devote+(boxlenth[i]-(tubheight-waterheight))*boxlenth[i]*boxlenth[i];
			else
			devote=devote+boxsize[i]*boxlenth[i]*boxlenth[i];
		}
	}
	if(devote<=(waterheight-watersize/tubarea)*tubarea)
	return true;
	else
	return false;
}