//Ilie Dumitru
#include<cstdio>

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    double sum=0, x[100001], y[100001];
    int i, N;
    scanf("%d", &N);
    for(i=0;i<N;++i)
        scanf("%lf%lf", x+i, y+i);
    fclose(stdin);
    *(x+N)=*x;
    *(y+N)=*y;
    for(i=0;i<N;++i)
        sum+=x[i]*y[i+1]-y[i]*x[i+1];
    if(sum<0)
        sum=-sum;
    printf("%lf", ((double)((long long int)(sum*5e4)))/1e5);
    fclose(stdout);
    return 0;
}