#include <cstdio>
#include <vector>

using namespace std;

#define NMAX 100009

long long total;
pair < int , int > A[NMAX];
int N,i;

int main()
{
freopen("aria.in","r",stdin);
freopen("aria.out","w",stdout);

for (i=1,scanf("%d",&N);i<=N;++i)
{
    scanf("%d %d",&A[i].first,&A[i].second);

    if (i==1)
    A[N+i]=A[i];
}

for (i=1;i<=N;++i)
total+=(1LL*A[i].first*A[i+1].second-1LL*A[i+1].first*A[i].second);

printf("%.8lf\n",1.0*total/2.0);

return 0;
}