#include<bits/stdc++.h>
#define rep(i,s,n) for(int i=s;i<n;i++)
using namespace std;
int main(){
  int n;
  cin>>n;
  int x[n],y[n];
  rep(i,0,n){
    cin>>x[i]>>y[i];
  }
  double ans=0;
  rep(i,1,n-1){
    ans+=abs((x[i]-x[0])*(y[i+1]-y[0])-(y[i]-y[0])*(x[i+1]-x[0]));
  }
  cout<<ans<<endl;
}