#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream c("aria.out");
int main()
{ signed long long x[10*10*10*10*10],y[10*10*10*10*10],N,s=0;
f>>N;
for(int i=1;i<=N;i++)
f>>x[i]>>y[i];
x[N+1]=x[1];
y[N+1]=y[1];
for(int i=1;i<=N;i++)
s+=x[i]*y[i+1]-x[i+1]*y[i];
c<<setprecision(6)<<fixed<<s/2;
}