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

if A*60<=B:
    print(-1)
    exit()

A*=60
C*=3600

OK=1<<63
NG=0

while OK>NG+1:
    mid=(OK+NG)//2

    if A*mid>=B*mid+C:
        OK=mid
    else:
        NG=mid

print(OK)