#include <iostream>
#include <cstdio>
#define cout cerr
#define MAX 100001
using namespace std;

pair <float, float> puncte[MAX];
int n;
float x,y;
float A;

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);

    scanf("%d ",&n);
    for(int i=1;i<=n;i++)
    {
        scanf("%f %f",&x,&y);
        puncte[i]=make_pair(x,y);
    }


    for(int i=1;i<n;i++)
        A+=puncte[i].first*puncte[i+1].second-puncte[i].second*puncte[i+1].second;

    A+=puncte[n].first*puncte[1].second-puncte[n].second*puncte[1].second;

    printf("%f",A);
    //cout<<A;
    return 0;
}