#include <cstdio>
#include <iomanip>
using namespace std;

int n;
double x[100003],y[100003];


FILE *fin,*fout;

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


    fscanf(fin,"%d",&n);
    for(int i=0; i<n; i++)
    {
        fscanf(fin,"%lf %lf",&x[i],&y[i]);
    }
    x[n]=x[0];
    y[n]=y[0];
    double aria=0.0;
    for(int i=0; i<n; i++)
    {
        aria+=((double)x[i]*y[i+1]-(double)x[i+1]*y[i]);
    }
    aria/=2.0;
    fprintf(fout,"%lf",aria);
    return 0;
}