#include <bits/stdc++.h>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

int n;
struct puncte{
 long double x , y;
}v[100005];
long double arie = 0;

int main()
{

f>>n;
for(int i=1;i<=n;i++){
 f>>v[i].x>>v[i].y;
}
v[n+1].x=v[1].x;
v[n+1].y=v[1].y;
for(int i=1;i<=n;i++){
 arie+= v[i].x * v[i + 1].y - v[i + 1].x * v[i].y;
}
arie/=2;
g<<arie;
}