#include <bits/stdc++.h>

using namespace std;

int n;
double x2,x1,y1,y2,s,x,y;
int main(){
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    cin>>n;
    cin>>x>>y;
    x1=x;
    y1=y;
    for (int i=2;i<=n;i++){
        cin>>x2>>y2;
        s+=0.5*(x1*y2-x2*y1);
        x1=x2;
        y1=y2;
    }
    s+=0.5*(x2*y-x*y2);
    cout<<fixed<<setprecision(5)<<s;
}