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

int main()
{
    int n;
    double x[100000], y[100000], aria=0;
    FILE *in=fopen("aria.in", "r");
    FILE *out=fopen("aria.in", "w");

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

    assert(1<=n && n<=100000);

    for(int i=0; i<n; i++)
    {
        fscanf(in,"%lf", &x[i]);
        fscanf(in,"%lf", &y[i]);
        assert(-1000000<=x[i] && x[i]<=1000000);
        assert(-1000000<=x[i] && x[i]<=1000000);

    }
    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;
}