#include <stdio.h>
int main()
{
    int n1,n2,n3;
    int ans;
    scanf("%d %d %d", &n1, &n2, &n3);
    ans=(((n3*(n3+1))/2)*n1)-n2;
    if(ans>0)
        printf("%d\n",ans);
    else
        printf("%d\n", 0);
    return 0;
}