import math
a,b,s=list(map(int,input().split ()))
n=0
for i in range(1,s+1):
    bm=min(max(math.floor(s/i),0),b)
    n+=int((a-i+1)*((b*bm)-(bm*(bm+1)*0.5)+1))
    if i==a:
        break
print(n)