#include <cstdio>
#include <fstream>

FILE *fin = fopen("aria.in","r");
std::ofstream fout("aria.out");

#define DIM 100010

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

int main()
{
    fscanf(fin,"%d",&n);
    for( i = 1 ; i <= n ; ++i )
    {
        fscanf(fin,"%d %d",&a[i],&b[i]);
    }

    for( i = 1 ; i < n ; ++i )
    {
        ans =+ a[ i ] * b[ i + 1 ] - b[ i ] * a[ i + 1 ] *1LL;
    }

    fout<<ans/2;


return 0;
}