#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct puncte
{
    long double x;
    long double y;
}p[100010];
long double aria;
int i,j,n;
int main()
{
f>>n;
for(i=1;i<=n;i++)
    f>>p[i].x>>p[i].y;
aria=0;
p[n+1]=p[1];
for(i=1;i<=n;i++)
{
    aria+=p[i].x*p[i+1].y-p[i+1].x*p[i].y;
}
    if(aria<0)aria=-aria;
    aria/=2.0;
    g<<fixed<<setprecision(6)<<aria;
}