#include<bits/stdc++.h>
using namespace std;
long long a,b,c;
bool check(long long x){
	long long ans=log10(x);
	ans+=x*a;
	return ans<=c; //小于等于说明买得起 
}
int main(){
	cin>>a>>b>>c;
	int l=0,r=1e9,mid,ans;
	while(l<=r){
		mid=(r-l)/2+l;
		if(check(mid)){ //如果是true,左指针右移 
			l=mid+1; ans=mid;
		}
		else{
			r=mid-1;
		}
	}
	cout<<ans;
return 0;
}