#include <stdio.h>
#include <stdlib.h>

int main()
{
    int n;
    float aria=0,x[1000000], y[1000000];

    FILE *F= fopen("aria.in","r");;
    FILE *G= fopen("aria.out","w");;

    fscanf(F,"%d", &n);


    for(int i=n; i>0; i--)
    {
        fscanf(F,"%f", &x[i]);
        fscanf(F,"%f", &y[i]);
    }

    for(int i=1; i<n; i++)
    {
        aria=aria+(1/2)*(x[i]*y[i+1]-x[i+1]*y[i]);
    }

    fprintf(G,"%.5f", aria);

    fclose(G);
    fclose(F);




    return 0;
}