#include<stdio.h>
void main()
{
    int pay;
    int bal;
    int act;
    scanf("%d %d %d",&pay,&bal,&act);
    if(bal>=pay)
    {
        printf(".2f\n",(bal-pay)*0.1);
    }else if(act>=0 || act<=1)
    {
        printf("%d");
    }else{
        printf("%d");
    }
    
}