#include <bits/stdc++.h>
using namespace std;
int i,j,n,m;
double X[100013];
double Y[100013];
double aria(0);
int main(void)
{
 ifstream cin("aria.in");
 ofstream cout("aria.out");
 cin>>n;
 for (i=1;i<=n;++i)
   cin>>X[i]>>Y[i];
 X[n+1]=X[1]; 
 Y[n+1]=Y[1];
 for (i=1;i<=n;++i) 
   aria+=(X[i]*Y[i+1] - X[i+1]*Y[i]);
 cout<<fixed<<setprecision(6)<<aria*0.500;
 return 0;
}