HDU-2036-JAVA-多边形面积

import java.io.*;
import java.util.*;

public class Q_2036 {
public static void main(String args[]){
Scanner in = new Scanner(new BufferedInputStream(System.in));
while(in.hasNext()){
int n = in.nextInt();
if(n==0){
break;
}
int x[] = new int
;
int y[] = new int
;
int s=0;
for(int i=0;i<n;i++){
x[i] = in.nextInt();
y[i] = in.nextInt();
}
for(int i=0;i<n-1;i++){
s += x[i]*y[i+1]-x[i+1]*y[i];
}
s += x[n-1]*y[0]-x[0]*y[n-1];
System.out.println(0.5*s);
}
}
}