#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <algorithm>
using namespace std;

#define PD pair<double, double>
#define x first
#define y second

int N;
PD V[100010];
double Ans;

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    int i;
    scanf("%i", &N);
    for(i = 1; i <= N; i++)
        scanf("%lf %lf", &V[i].x, &V[i].y);
    V[N + 1] = V[1];
    for(i = 1; i <= N; i++) Ans += V[i].x * V[i + 1].y - V[i + 1].x * V[i].y;
    printf("%.6lf\n", fabs(Ans / 2.0));
    return 0;
}