#include <bits/stdc++.h>
using namespace std;
pair<int,int> v[100005];
int main(){
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    int n;
    scanf("%d",&n);
    long long S=0;
    for(int i=1;i<=n;++i)
        scanf("%d %d",&v[i].first,&v[i].second);
    for(int i=1;i<n;++i)
        S+=1LL*v[i].first*v[i+1].second-1LL*v[i+1].first*v[i].second;
    S+=1LL*v[n].first*v[1].second-1LL*v[1].first*v[n].second;
    double ans=S/2;
    printf("%.1f\n",ans);
    return 0;
}