[백준] 2166번 : 다각형의 면적 - C

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

#include<stdio.h>
#include<math.h>
#include<stdlib.h>

int main() {
    int n;
    scanf("%d",&n);
    double num=0;
    double x1,y1;
    scanf("%lf %lf",&x1,&y1);
    double x0=x1;
    double y0=y1;
    for(int i=0;i<n-1;i++){
        double x2,y2;
        scanf("%lf %lf",&x2,&y2);
        num+=x1*y2-x2*y1;
        if(i==n-2) num+=x2*y0-x0*y2;
        x1=x2;
        y1=y2;
    }
    printf("%.1lf",fabs(num)/2.0);
}