zoj 3693

#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\n",s+0.01*k);

}

return 0;

}