#include <iostream>
#include <fstream>
#include <cstdio>
using namespace std;
ifstream fin("aria.in");
FILE *fout=fopen("aria.out","w");
int n,i;
float s,x[100001],y[100001];
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++)
        s+=(x[i]*y[i+1]-x[i+1]*y[i]);
    s/=2;
    fprintf(fout,"%.5f",s);
    return 0;
}