#include<bits/stdc++.h>
using namespace std;

int n;
const int maxN=(1e5)+5;

pair<float,float> v[maxN];

float S;

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

    scanf("%d",&n);

    for(int i=1;i<=n;i++)
        scanf("%f%f",&v[i].first,&v[i].second);

    v[n+1]=v[1];

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

    S*=0.5;

    if(S<0.0) S=-S;

    printf("%.5f\n",S);

    return 0;
}