#include<bits/stdc++.h>
using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");


const int NMAX=100005;

int n;
long double a[NMAX],b[NMAX];
long double sol;

int main()
{
    int i;
    long double aux;
    fin>>n;
    for (i=1;i<=n;i++) fin>>a[i]>>b[i];
    for (i=1;i<n;i++)
        sol=sol+(1LL*a[i]*b[i+1]-1LL*a[i+1]*b[i])/2;
    sol=sol+(1LL*a[n]*b[1]-1LL*a[1]*b[n])/2;
    if (sol<0) sol=sol*(-1.0);
    cout<<setprecision(5)<<fixed;
    fout<<sol<<"\n";
    return 0;
}