#include <stdio.h>
#include <math.h>

int main()
{
    FILE *fin, *fout;
    int n, x0, y0, xc, yc, xurm, yurm;
    double A = 0;

    fin = fopen("aria.in", "r");
    fout = fopen("aria.out", "w");

    if (fin == NULL || fout == NULL)
        return 1;

    fscanf(fin, "%d %d %d", &n, &x0, &y0);

    xc = x0;
    yc = y0;

    for (int i = 2; i <= n; i++) {
        fscanf(fin, "%d %d", &xurm, &yurm);

        A += (double)xc * yurm - (double)yc * xurm;

        xc = xurm;
        yc = yurm;
    }

    A += (double)xc * y0 - (double)yc * x0;

    A = fabs(A) / 2.0;

    fprintf(fout, "%.1f", A);

    fclose(fin);
    fclose(fout);

    return 0;
}