#include <cstdio>
using namespace std;

#define NMax 100005

int n;
double A;

struct coord
{
    double first, second;
} v[NMax];

int main()
{
    FILE *f,*g;
    f = fopen("aria.in","rt");
    g = fopen("aria.out","wt");

    int i;

    fscanf(f,"%d",&n);
    for(i=1;i<=n;++i) fscanf(f,"%lf %lf\n",&v[i].first,&v[i].second);
    v[n+1] = v[1];

    for(i=1;i<=n;++i) A += 0.5 * (v[i].first * v[i+1].second - v[i+1].first * v[i].second);

    fprintf(g,"%.5lf\n",A);

    fclose(f);
    fclose(g);
    return 0;
}