{programmer:mogtaba amirzade 
url http:\\www.ifjam.com
}
PROGRAM P6(INPUT,OUTPUT);
VAR {12}
S,I,N:INTEGER;
BEGIN
READLN(N);
S:=0;
I:=0;
REPEAT
S:=S+I;
I:=I+1;
UNTIL I>N;
WRITELN(S);
READLN
END.