HDOJ2036(多边形面积)

#include <iostream> 
#include <cstring>
#include <string>
#include <cstdio>
#include <algorithm>
#include <stack>
#include <math.h>

using namespace std;

int a[101];
int b[101];

int main()
{
int n;
while (cin >> n, n)
{
for (int i = 0; i < n; i++)
{
cin >> a[i] >> b[i];
}
a[n] = a[0];
b[n] = b[0];
double sum = 0;
for (int i = 0; i < n; i++) sum += fabs(a[i] * b[i + 1] - a[i + 1] * b[i]);
printf("%.1lf\n",sum/2.0);
}
}