#include<bits/stdc++.h>
using namespace std;
int main(){
	long long n,x,y,total=0;
	cin>>x>>y>>n;
	x--;
	y--;
	while(n>=y-1){
		total+=y;
		n-=10;
	}
	total+=n*x;
	cout<<total;
	return 0;
}