#include <stdio.h>
#define nmax 100005

using namespace std;

int main() {
    freopen ("aria.in", "r", stdin);
    freopen ("aria.out", "w", stdout);
    int N, x[nmax], y[nmax], i;
    float sum = 0;
    scanf ("%d", &N);
    for (i = 1; i <= N; ++i)
        scanf ("%d %d", &x[i], &y[i]);
    x[N+1] = x[1];
    y[N+1] = y[1];
    for (i = 1; i <= N; ++i)
        sum += x[i] * y[i+1] - x[i+1] * y[i];
    sum = sum/2.0;
    printf ("%.5f", sum);
    return 0;
}