#include<iostream>
#include<iomanip>
#include<complex>
using namespace std;
typedef complex<int> xy_t;

xy_t P[101];

double cross(xy_t p,xy_t q){
    return (conj(p)*q).imag();
}

int main(){
    int n;
    cin>>n;
    int x,y;
    for(int i=0;i<n;++i){
        cin>>x>>y;
        P[i]=xy_t(x,y);
    }

    double S=0.0;
    for(int i=0;i<n-2;++i){
        S+=cross(P[i+1]-P[0],P[i+2]-P[0])/2;
    }
    cout<<fixed<<setprecision(1)<<S<<"\n";
}