#include<fstream>
#include<math.h>
#include<iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
double a,b,xa,ya,xi,yi,n,d,s=0;
int main()
{
    fin>>n>>xa>>ya;
    a=xa;
    b=ya;
    while(fin>>xi>>yi)
    {
        d=a*yi-xi*b;
        s=s+d;
        a=xi;
        b=yi;
    }
    d=a*ya-xa*b;
    s=s+d;
    if(s<0) s=-s;
    s/=2;
    fout<<s;
    fin.close();
    fout.close();
    return 0;
}