#include <cmath>
#include <stdio.h>
#define N 100005
using namespace std;

typedef struct {double x,y;} PUNCT;

int n,i;
double rez=0;
PUNCT A[N];

FILE *f,*g;

int main()
{
    f=fopen("aria.in","r");
    g=fopen("aria.out","w");

    fscanf(f,"%d",&n);
    for (i=0;i<n;++i)
        fscanf(f,"%lf %lf",&A[i].x,&A[i].y);
    A[n]=A[0];
    for (i=0;i<n;++i)
        rez=rez+(A[i].x*A[i+1].y-A[i+1].x*A[i].y);
    fprintf(g,"%lf",(rez/2));
    fclose(f);
    fclose(g);
    return 0;
}