#include<bits/stdc++.h>
using namespace std;
long long a,b,c;
inline bool check(long long k)
{
	return (k*a+(floor(log10(k))+1)*b)<=c;
}
int main()
{
	scanf("%lld%lld%lld",&a,&b,&c);
	long long l=0,r=1e9;
	while(l<r)
	{
		long long mid=(l+r+1)/2;
		if(check(mid)) l=mid;
		else r=mid-1;
	}
	printf("%lld\n",l);
	return 0;
}