#include <cstdio>
#include <cstdlib>
#include <utility>
using namespace std;

int n;

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);

    pair<float, float> start, points[2];
    double sol = 0;

    scanf("%d", &n);
    scanf("%f %f", &start.first, &start.second);
    points[1] = start;
    for (int i=1; i<n; ++i){
        points[0] = points[1];
        scanf("%f %f", &points[1].first, &points[1].second);
        sol += points[0].first*points[1].second-points[1].first*points[0].second;
    }
    sol += points[1].first*start.second-points[1].second*start.first;
    printf("%.5f", sol>0 ? sol/2 : -sol/2);
    return 0;
}