#include <iostream>
using namespace std;

int main()
{
	int T, S, P;
	cin >> T >> S >> P;
	float dis[5] = {0.5, 0.6, 0.8, 0.9, 0};
	int ans = 0;
	for(int i=0; i<5; i++)
	{
		int cnt = int(T * 0.2);
		if(i == 0) cnt += 1;
		cnt = min(cnt, S);
		ans += int(P * dis[i]) * cnt;
		S -= cnt;
		if(S == 0) break;
	}
	cout << ans << endl;
}