View Code of Problem 4049

#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <queue>
#include <vector>
using namespace std;

int main(void)
{
    int n;
    while (cin >> n && n != 0){
        double ans = 0;
        double pos[n][2];
        for (int i = 0; i < n;i++){
            cin >> pos[i][0] >> pos[i][1];
        }
        for (int i = 0; i < n;i++){
            ans += (pos[i][0] * pos[(i + 1) % n][1]) - (pos[(i + 1) % n][0] * pos[i][1]);
        }
        ans /= 2;
        printf("%.1lf\n",ans);
    }
}