#include<stdio.h>
#include<stdlib.h>
int main(void) {
	int A, B, X, a = 0, b = 0, x, aiu = 0, minimum = 100000000;
	scanf("%d", &A);
	scanf("%d", &B);
	scanf("%d", &X);
	for (a = 0; a <= 40;a++) {
		for (b = 0; b <= 20;b++) {
			x = a * 500 + b * 1000;
			aiu = A*b + B*a;
			if (X <= x && minimum>aiu) {
				minimum = aiu;
			}
		}
	}
	printf("%d\n", minimum);
	return 0;
}
