program zadacha;

uses crt;

var a,b,P: real;

BEGIN

clrscr;

writeln('vvedi perimetr'); readln(P); clrscr;

writeln('vvedi storonu'); readln(a);  clrscr;

b:=(P-2*a):2;

writeln('vtoraia storona ravna');

writeln(b);

readln;

end.