#include <stdio.h>
double fn( double x, int n );
int main()
{
double x;
int n;
scanf("%lf %d", &x, &n);
printf("%.2f\n", fn(x,n));
return 0;
}
/* 你的代码将被嵌在这里 /
double fn( double x, int n )
{
double result,sum=0;
if(n==1)
result=x;
else
{
result=xfn(x,n-1);
}
(n%2)==0?-result:result;
sum=sum+result;
return sum;
}