#include <cstdio>

using namespace std;

int v[100010][2];

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int n,s=0;
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
        scanf("%d%d",&v[i][1],&v[i][2]);
    v[n+1][1]=v[1][1];
    v[n+1][2]=v[1][2];
    for(int i=1;i<=n;i++)
        s=s+(0-v[i][1])*(0-v[i+1][2])-(0-v[i+1][1])*(0-v[i][2]);
    printf("%f",s/2.0);
    return 0;
}