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

#define Max 100005

struct punct {
    double x, y;
} vec[Max];


int main()
{

    int N;
    long double  aria=0;
    FILE *in=fopen("aria.in", "r");
    FILE *out=fopen("aria.in", "w");

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

    for(int i=0; i<N; i++)
    {
        fscanf(in,"%lf", &vec[i].x);
        fscanf(in,"%lf", &vec[i].y);

    }
    vec[N]=vec[0];

    for(int i=0; i<N; i++)
    {
        aria=aria+(vec[i].x*vec[i+1].y-vec[i+1].x*vec[i].y);
    }

    fprintf(out,"%lf", fabs(aria/2));


    fclose(out);
    fclose(in);


    return 0;
}