#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
#define MAXN 100005
struct pc
{
    double x,y;
}v[MAXN];

double calc_arie(pc A, pc B, pc C)
{
    double x = (B.x-A.x)*(C.y-A.y)-(C.x-A.x)*(B.y-A.y);
    return abs(x) / 2 ;
}
int n,i;
double aria;
int main()
{
    cin>>n;
    for(i=1; i<=n; i++)
        cin>>v[i].x>>v[i].y;

    for(i=2; i<=n-1; i++)
        aria+=calc_arie(v[1],v[i],v[i+1]);

    cout<<aria;
}