#include <cstdio>
using namespace std;
int x[100005],y[100005];
int main()
{
    int i,n;
    long long arie=0;
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d\n",&n);
    scanf("%d %d\n",&x[1],&y[1]);
    for (i=2;i<=n;++i)
    {
        scanf("%d %d\n",&x[i],&y[i]);
        arie+=((x[i-1]*y[i])-(x[i]-y[i-1]));
    }
    arie+=((x[n]*y[1])-(x[1]-y[n]));
    printf("%lld",(arie));
    if (arie&1)
        printf(".5");
    printf("\n");
    fclose(stdin);
    fclose(stdout);
    return 0;
}