#include <cstdio>
#include <utility>
#include <cmath>
#define MAX 100

int main(void)
{

    int n;
    std::pair<double, double> point[MAX];

    scanf("%d", &n);
    for(int i = 0 ; i < n ; ++i)
    {
        scanf("%lf %lf", &point[i].first, &point[i].second);
    }

    double ans = 0;
    for(int i = 0 ; i < n ; ++i)
    {
        if(i + 1 == n)
        {
            ans += (point[i].first - point[0].first) * (point[i].second + point[0].second);
        }else
        {
            ans += (point[i].first - point[i + 1].first) * (point[i].second + point[i + 1].second);
        }
    }
    printf("%.1lf\n", ans / 2);

    return 0;
}