#include <bits/stdc++.h>
#define x first
#define y second
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n,s,d,x,nr,i;
double sum;
pair<double,double> ant,primul,act;
double det(pair<double,double> p1,pair<double,double> p2){
/// 0   0   1
/// p1x p1y 1
/// p2x p2y 1
/// p1x*p2y-p1y*p2x
return p1.x*p2.y/2.0 - p1.y*p2.x/2.0;
}
int main(){
fin>>n;
fin>>ant.x>>ant.y;
primul=ant;
for(i=2;i<=n;i++){
    fin>>act.x>>act.y;
    sum+=det(ant,act);
    ant=act;
}
sum+=det(ant,primul);
fout<<setprecision(5)<<sum;
}