#include <stdio.h>
#include <stdlib.h>

int main()
{
    int aria,n, x[100000], y[100000];

    FILE *in;
    FILE *out;
    in=fopen("aria.in", "r");;
    out=fopen("aria.out", "w");;

    fscanf(in,"%d", &n);

    for(int i=0; i<n; i++)
    {
        fscanf(in,"%d", &x[i]);
        fscanf(in,"%d", &y[i]);
    }

    for(int i=0; i<n; i++)
    {
        aria=1/2*(x[i]*y[i+1]-x[i+1]*y[i]);
    }

    fprintf(out,"%d", aria);

    fclose(out);
    fclose(in);




    return 0;
}