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

#define Max 100005



int main()
{

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

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

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


    }
    x[N]=x[0];
    y[N]=y[0];

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

    fprintf(out,"%lf", fabs(aria/2.0));


    fclose(out);
    fclose(in);


    return 0;
}