#include<cstdio>
#define NMAX 100000+5
using namespace std;
int N,i;
double S,x[NMAX],y[NMAX];
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&N);
    for(i=1;i<=N;i++)
        scanf("%lf%lf",&x[i],&y[i]);
    x[i]=x[1];
    y[i]=y[1];
    for(i=1;i<=N;i++)
        S+=x[i]*y[i+1]-x[i+1]*y[i];
    S=0.5*S;
    printf("%.5lf",S);
    return 0;
}