#include <cstdio>
#include <cmath>
using namespace std;
FILE*f=fopen("aria.in","r");
FILE*g=fopen("aria.out","w");
struct pct{double x,y;} vec[100005];
int n,i;
double sol;
int main()
{
    fscanf(f,"%d",&n);
    for(i=0;i<n;i++)
        fscanf(f,"%lf %lf",&vec[i].x,&vec[i].y);
    vec[n]=vec[0];
    for(i=0;i<n;i++)
        sol+=(vec[i].x*vec[i+1].y-vec[i].y*vec[i+1].x);
    fprintf(g,"%lf",fabs(sol/2.0));
    return 0;
}