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AreaOfPolygon.java
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AreaOfPolygon.java
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import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.StringTokenizer;
public class AreaOfPolygon {
private static long[][] resultArr;
public static void main(String[] args) throws Exception {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
StringTokenizer st;
int N = Integer.parseInt(br.readLine());
initResultArr(N);
for(int i = 0; i < N; i++) {
st = new StringTokenizer(br.readLine(), " ");
int x = Integer.parseInt(st.nextToken());
int y = Integer.parseInt(st.nextToken());
resultArr[i][0] = x;
resultArr[i][1] = y;
}
double area = getArea();
System.out.printf("%.1f", area);
}
private static void initResultArr(int length) {
resultArr = new long[length][2];
}
private static double getArea() {
long plusSum = 0;
long minusSum = 0;
for(int i = 0; i < resultArr.length; i++) {
plusSum += getPlusValue(i);
minusSum += getMinusValue(i);
}
return Math.abs(plusSum - minusSum) / 2.0;
}
private static long getPlusValue(int index) {
if(index == resultArr.length - 1) return resultArr[index][0] * resultArr[0][1];
return resultArr[index][0] * resultArr[index+1][1];
}
private static long getMinusValue(int index) {
if(index == resultArr.length - 1) return resultArr[index][1] * resultArr[0][0];
return resultArr[index][1] * resultArr[index+1][0];
}
}