#include<bits/stdc++.h>
using namespace std;	
int main () {
	long long x,y,n;
	cin >> x >> y >> n;
	x--;
	y--;
	long long sum = 0;
	long long max_ = max(x,y);
	long long min_ = min(x,y);
	int z;
	if (min_ == x) {
		z = 1;
	}
	else {
		z = 10;
	}
	while (n >= z) {
		n -= z;
		sum += min_;
	}
	sum += n * max_;
	cout << sum << endl;
	return 0;
}

所以，为什么会90？