#include<cstdio>
using namespace std;
int n,i;
double A,x[100009],y[100009];
int main()
{
freopen("aria.in","r",stdin);
freopen("aria.out","w",stdout);
scanf("%d",&n);
for(i=1;i<=n;i++)
{
    scanf("%lf",&x[i]);
    scanf("%lf",&y[i]);
}
x[n+1]=x[1];y[n+1]=y[1];
A=0;
for(i=1;i<=n;i++)
    A=(double)A+x[i]*y[i+1]-y[i]*x[i+1];
if(A<0) A*=-1;
printf("%.5lf\n",(double)A/2);
return 0;
}