#include<stdio.h>
#include<string.h>//进位问题如3.985 应该进位3.99
int main() {
int n,k,i;
double w,s;
char str[30];
while(scanf("%d%lf%d",&n,&w,&k)!=EOF) {
i=(n+2)/k;
n=n+2-i;
s=n*w/2;
sprintf(str,"%lf",s);
i=0;
while(str[i]!=0&&str[i]!='.')
i++;
k=0;
if(str[i+3]>='5')
k=1;
str[i+3]=0;
sscanf(str,"%lf",&s);
printf("%.2f
",s+0.01*k);
}
return 0;
}