#include<bits/stdc++.h>
#define maxN 100005
using namespace std;
int n,x,y,r,l;
pair<int,int> v[maxN];
long double aria;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);

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

    for(int i=1;i<=n;i++)
    {
        r=i+1;
        if(r>n) r=1;
        l=i;
        aria=aria+(long double)fabs(0.5*(v[l].first*v[r].second-v[r].first*v[l].second));
    }

    printf("%.7llf\n",aria);

    return 0;
}