#include <cstdio>

using namespace std;

const char Input[]="aria.in";
const char Output[]="aria.out";
const int DIMN=100010;

int n;
double t,x[DIMN],y[DIMN];

int main()
{
    double x3,y3;
    freopen(Input,"r",stdin);
    freopen(Output,"w",stdout);
    scanf("%d",&n);
    for(int i=0;i<n;++i)
        scanf("%lf %lf",&x[i],&y[i]);
    x[n]=x[0];
    y[n]=y[0];
    t=0;
    for(int i=0;i<n;++i)
        t+=x[i]*y[i+1]-y[i]*x[i+1];
    t/=2;
    if(t<0)
        t*=-1;
    printf("%.7lf\n",t);
    return 0;
}