#include <bits/stdc++.h>
using namespace std;
#define int double
signed main()
{
    ifstream cin("aria.in");
    ofstream cout("aria.out");
    int n;
    cin>>n;
    int x,y,a,b,x2,y2;
    cin>>x>>y;
    a=x;
    b=y;
    int ans=0;
    for(int i=2;i<=n;i++){
        cin>>x2>>y2;
        ans=ans+x2*y-x*y2;
        x=x2;
        y=y2;
    }
    ans=ans+a*y-x*b;
    ans=abs(ans);
    ans=ans/2;
    cout<<fixed<<setprecision(5)<<ans;
    return 0;
}