公式：
#include <stdio.h>
#include <stdlib.h>
int main()
{
int n;
int x[1000];
int y[1000];
while (scanf("%d", &n) && n != 0)
{
int sum = 0;
double ans = 0;
for (int i = 1; i <= n; i++)
{
scanf("%d %d", &x[i], &y[i]);
}
for (int k = 1; k <= n - 1; k++)
{
sum = sum + (x[k] * y[k + 1] - x[k + 1] * y[k]);
}
sum = sum + (x[n] * y[1] - y[n] * x[1]); //不要忘记(Xn Yn)和(X1 Y1)
ans = 0.5*abs(sum); //abs只能对整数求绝对值
printf("%.1f\n", ans);
}
}
