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