Codeforces 546A

#include <stdio.h>

int main()
{
	int k, n , w;
	scanf("%d%d%d", &k, &n, &w);
	int ans=(k*((w*(w+1))/2))-n;
	if(ans>0)
		printf("%d\n", ans);
	else
		printf("%d\n", 0);
	return 0;
}