[백준] 2166번 : 다각형의 면적 - Java(자바)

https://www.acmicpc.net/problem/2166

import java.util.Scanner;
public class Main {
    public static void main(String args[]) {
        Scanner s=new Scanner(System.in);
        int n=s.nextInt();
        long num=0;
        long x1=s.nextInt(), y1=s.nextInt();
        long x0=x1;
        long y0=y1;
        for(int i=0;i<n-1;i++){
            long x2=s.nextInt(), y2=s.nextInt();
            num+=x1*y2-x2*y1;
            if(i==n-2) num+=x2*y0-x0*y2;
            x1=x2;
            y1=y2;
        }
        System.out.format("%.1f",Math.abs(num)/2.0);
    }
}