#include<iostream> // cin\cout\endl #include<cstdio> //scanf()\printf() #include<cstring> // strcpy()\strcat()\strcmp()\strlen()\memset() #include<cmath> //sqrt()\pow()\abs()\ceil()\floor()\max()\min() using namespace std; int main(){ //a-s*d int a,s,d,l; cin>>a>>s>>d; if(d/s%2==1){ l=d/s+1; }else{ l=l+0; } if(a-l<0){cout<<0;} else{ cout<<a-l; } return 0; }