WA點: 把P也當浮點數輸入了...wa test1 一次...
#include<iostream>
#include<cstdio>
#include<string>
#include<cstring>
#include<algorithm>
#include<cmath>
using namespace std;
const double eps=1e-8;
const double inf=1e10;
double X[15005];
int P[15005];
int N;
int sgn(double x)
{
return x<-eps?-1:x>eps;
}
double Fabs(double x)
{
return (sgn(x)<0)?-x:(x>eps?x:0);
}
double function(double x)
{
double ret=0;
int i;
for(i=0;i<N;++i)
{
ret+=Fabs(X[i]-x)*P[i];
}
return ret;
}
double work()
{
double l=-1.0,r=50005.0;
double m,tm;
while((r-l)>eps)
{
m=(l+r)/2;
tm=(m+r)/2;
// m=(l+tm)/2;
if(function(m)<function(tm))
r=tm;
else
l=m;
}
return l;
}
int main()
{
int i;
while(scanf("%d",&N)!=EOF)
{
for(i=0;i<N;++i)
{
scanf("%lf %d",&X[i],&P[i]);
}
// for(i=0;i<N;++i)
// printf("%f %f\n",X[i],P[i]);
printf("%.5f\n",work());
}
return 0;
}