#include<fstream>
#include<iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n,m,i,j;
double sum,x[100005],y[100005];
int main(){
    fin>>n;
    for(i=1;i<=n;i++){
        fin>>x[i]>>y[i];
    }
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(i=1;i<=n;i++){
        if(sum>=0)
            sum-=(x[i+1]*y[i])/2;
        else
            sum+=(x[i]*y[i+1])/2;
    }
    fout<<setprecision(5)<<fixed<<sum;

    return 0;
}