#include <bits/stdc++.h>
using namespace std;
struct point 
       {
       	int x,y;
	   } T[100013];
int n,i,j;
double aria(0);

int main(void)
{
 ifstream cin("aria.in");
 ofstream cout("aria.out");
 cin>>n; 
 for (i=1;i<=n;++i)
   	cin>>T[i].x>>T[i].y;
 T[n+1].x=T[1].x;
 T[n+1].y=T[1].y;
 for (i=1;i<=n;++i)
  aria+=(T[i].x*T[i+1].y-T[i+1].x*T[i].y);              
 cout<<aria/2;
 return 0;
}