#include <cstdio>
#define NMAX 100010

using namespace std;

FILE *fin,*fout;

int n,a[NMAX],b[NMAX],S;
double cat;

int main()
{
    int i;
    fin=fopen("aria.in","r");
    fout=fopen("aria.out","w");
    fscanf(fin,"%d",&n);
    for(i=1;i<=n;i++)
        fscanf(fin,"%d%d",&a[i],&b[i]);
    for(i=1;i<=n;i++)
    {
        if(i!=n)
        {
            S=S+(a[i]*b[i+1]-a[i+1]*b[i]); //sa nu uit la afisare sa pun /2
        }
        else
        {
            S=S+(a[n]*b[1]-a[1]*b[n]);
        }
    }
    cat=S;
    if(S%2==0)
        fprintf(fout,"%0.0f",cat/2);
    else
        fprintf(fout,"%0.1f",cat/2);
    return 0;
}