暴力列举的为啥能错

#include<bits/stdc++.h>
using namespace std;
int main()
{
	long long a,b,c,d,minn=9999999999999,sum;
	cin>>a>>b>>c;
	d=c/10;
	if(d==0)
	{
		cout<<c*(a-1);
	}
	else
	{
		for(long long f=0;f<=d;f++)
		{
			sum=f*(b-1)+(c-10*f)*(a-1);
			minn=min(sum,minn);
		}
		cout<<minn;
	}
		return 0;
}