Var
    XPrev, YPrev, Sq, X, Y, SX, SY : Int64;
    N : LongInt;
Begin
    Read(N);
    Read(XPrev, YPrev);
    SX := XPrev; SY := YPrev;
    Sq := 0;
    For N := 2 To N Do
    Begin
        Read(X, Y);
        Inc(Sq, (XPrev - X) * (Y + YPrev));
        XPrev := X;
        YPrev := Y;
    End;
    Inc(Sq, (X - SX) * (Y + SY));
    Sq := Abs(Sq);
    WriteLn(SQ Div 2, '.', (Sq Mod 2) * 5);
End.