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

using namespace std;



int N; double x[100005],y[100005];

void cit()
{
    scanf("%d\n",&N);

    for (int i=1;i<=N;i++)
        scanf("%lf %lf\n",&x[i],&y[i]);
}

void aria()
{
    double arie=0;
    int i;
    x[N+1]=x[1];
    y[N+1]=y[1];
    for (i=1;i<=N;i++)
        arie+=(x[i]*y[i+1] - x[i+1]*y[i]);
    printf("%lf\n",arie/2);
}

int main()
{
    freopen ("aria.in","r",stdin);
    freopen ("aria.out","w",stdout);

    cit();
    aria();

    fclose(stdin);
    fclose(stdout);

    return 0;
}