#include <cstdio>

using namespace std;

int n, i;
double x[100007], y[100007], a;

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

    scanf("%d", &n);

    for(i = 1; i <= n; i++)
        scanf("%lf %lf", &x[i], &y[i]);

    x[n+1] = x[1];
    y[n+1] = y[1];

    for(i = 1; i <= n; i++)
        a += x[i]*y[i+1] - x[i+1]*y[i];

    printf("%.5lf", a/2.0);

}