#include <iostream>
#include <cstdio>

using namespace std;

int n;
long long x0,y0,x,y,A,A_tot;
pair <long long , long long > v[100005];

void citire ()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);

    scanf("%d",&n);

    for (int i=1; i<=n; ++i)
    {
        scanf("%lld%lld",&x,&y);
        v[i].first=x;
        v[i].second=y;
    }


}



int main()
{
    citire();

    v[0]=v[n];

    for (int i=1;i<=n;++i)
        A_tot+=(v[i-1].first-v[i].first)*(v[i].second+v[i-1].second)/2;

    printf("%lld",A_tot);
    return 0;
}