#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;

float Arie(int xa,int ya, int xb,int yb)
{
    return xa*yb-ya*xb;
}

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+=Arie(puncte[i].first,puncte[i].second,puncte[i+1].first,puncte[i+1].second);
        //cout<<puncte[i].first*puncte[i+1].second-puncte[i].second*puncte[i+1].second<<" __ ";
       // cout<<puncte[i].first<<" "<<puncte[i].second<<" ~ "<<puncte[i+1].first<<" "<<puncte[i+1].second<<endl;
    }
    A+=puncte[n].first*puncte[1].second-puncte[n].second*puncte[1].second;

    if(A>0)
        printf("%f",A/2);
    else  printf("%f",-A/2);
    return 0;
}