23 lines
488 B
C++
23 lines
488 B
C++
#include <math.h>
|
|
#include <stdio.h>
|
|
int main(void)
|
|
{
|
|
int x[3], y[3], n;
|
|
double sum;
|
|
while (scanf("%d", &n), n)
|
|
{
|
|
scanf("%d%d", x, y);
|
|
x[2] = x[0]; y[2] = y[0];
|
|
sum = 0.0;
|
|
while (--n)
|
|
{
|
|
scanf("%d%d", x+1, y+1);
|
|
sum += x[0]*y[1] - x[1]*y[0];
|
|
x[0] = x[1]; y[0] = y[1];
|
|
}
|
|
sum += x[0]*y[2] - x[2]*y[0];
|
|
printf("%.1f\n", sum / 2.0);
|
|
}
|
|
return 0;
|
|
}
|