#include <stdio.h>

int main()
{
	int K, N, F, a;
	int *A;
	scanf("%d%d%d", &K, &N, &F);
	A = (int *)malloc(F);
	a = K * N;
	for (int i = 0; i < F; i++)
	{
		a -= *A;
		A++;
	}
	if (a < 0)
		a = -1;
	printf("%d\n", a);
	return 0;
}