26 lines
544 B
C
26 lines
544 B
C
#include <stdio.h>
|
|
#define ABS(x) ((x>0)?(x):(-x))
|
|
|
|
struct point{
|
|
long x;
|
|
long y;
|
|
};
|
|
|
|
int main(void){
|
|
int numbers = 0;
|
|
struct point points[128];
|
|
while(scanf("%d",&numbers) == 1 && numbers){
|
|
int i;
|
|
double s = 0.0;
|
|
for(i = 0; i < numbers; i++){
|
|
scanf("%ld",&points[i].x);
|
|
scanf("%ld",&points[i].y);
|
|
}
|
|
for(i = 0; i < numbers-1; i++)
|
|
s += points[i].x * points[i+1].y - points[i].y * points[i+1].x;
|
|
s += points[numbers-1].x * points[0].y - points[numbers-1].y * points[0].x;
|
|
printf("%.1lf\n",0.5*ABS(s));
|
|
}
|
|
return 0;
|
|
}
|