#include <cstdio>

using namespace std;

const int MAX=100005;
double x[MAX],y[MAX],aria;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int n,i;
    scanf("%d",&n);
    for(i=1;i<=n;++i)
        scanf("%lf%lf",x+i,y+i);
    x[n+1]=x[1],y[n+1]=y[1];
    for(i=1;i<=n;++i)
        aria+=(x[i]*y[i+1]-x[i+1]*y[i]);
    aria/=2;
    printf("%.5lf",aria);
    return 0;
}