#include <stdio.h>
#include <math.h>
FILE *fin, *fout;
fin = fopen("aria.in", "r");
fout = fopen("aria.out", "w");
int main()
{
    int n, x0, y0, xc, yc, xurm, yurm, A = 0;
    fscanf("%d %d %d", &n, &x0, &y0);
    xc = x0, yc = y0;
    for(int i = 2; i <= n; i++){
        fscanf("%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("%d", A);
    return 0;
}