#include <iostream>
#include <fstream>
#define NMAX 100005
using namespace std;
long double x1, y1, x2, y2, firstx, firsty, aria;
int N;

int main()
{
    freopen("aria.in","rt",stdin);
    freopen("aria.out","wt",stdout);
    scanf("%d%Lf%Lf", &N, &x1, &y1);
    firstx=x1;
    firsty=y1;
    for(int i=2;i<=N;i++){
        scanf("%Lf%Lf", &x2, &y2);
        aria+= x1*y2 - x2*y1;
        x1=x2;
        y1=y2;
    }
    aria+= firstx*y2 - x2*firsty;
    cout<<aria<<'\n';
    return 0;
}