#include<cstdio>
#include<algorithm>
#include<cmath>
using namespace std;
int i,n;
double x[100010],y[100010],Aria;
inline double det(int I,int J,int K)
{
    return x[I]*y[J] + x[J]*y[K] + y[I]*x[K] - y[J]*x[K] - y[K]*x[I] - y[I]*x[J];
}
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]);
    for(i=2;i<n;i++)
        Aria += det(1,i,i+1);
    printf("%lf",fabs(Aria/2.0));
    return 0;
}