import sys
input = sys.stdin.readline

A,B,S=map(int,input().split())

ANS=0

for tate in range(1,S+1):
    yokomax=min(B,S//tate)
    #print(tate,yokomax)

    tax=A-(tate-1)
    if tax<=0:
        continue

    yox=B*yokomax-yokomax*(yokomax-1)//2
    if yox<=0:
        continue

    ANS+=tax*yox
    #print(tax,yox,ANS)

print(ANS)