#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;
#define MAXN 100005
struct pc
{
    double x,y;
}v[MAXN];

double calc_arie(pc A, pc B, pc C)
{
    double x = (B.x-A.x)*(C.y-A.y)-(C.x-A.x)*(B.y-A.y);
    return x / (double) 2 ;
}
int n,i;
double aria;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d", &n);
    for(i=1; i<=n; i++)
         scanf("%lf %lf", &v[i].x, &v[i].y);

    for(i=2; i<=n-1; i++)
        aria+=calc_arie(v[1],v[i],v[i+1]);

     printf("%.5lf", aria);
}