#include <cstdio>
#include <fstream>
#include <iomanip>

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

#define DIM 100010

int n,a[DIM],b[DIM],i;
long double ans;

int main()
{
    fin>>n;
    for( i = 1 ; i <= n ; ++i )
    {
        fin>>a[i]>>b[i];
    }

    a[ n + 1 ] = a[ 1 ];
    b[ n + 1 ] = b[ 1 ];

    for( i = 1 ; i <= n ; ++i )
    {
        ans += a[ i ] * b[ i + 1 ] - b[ i ] * a[ i + 1 ] *1LL;
    }
    fout.precision(5);
    ans = ans / 2;
    fout<<ans;


return 0;
}