code:

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int main(){
    ll a,b,x;
    cin>>a>>b>>x;
    ll l=1,r=1e18;
    ll ans=0,mid;
    while(l<r){
        mid=(l+r)>>1;
        if(a*mid+b*int(log10(mid))+b<=x){ans=mid;l=mid+1;}
        else r=mid;
    }
    cout<<ans<<endl;
}