from collections import defaultdict,deque
import sys,heapq,bisect,math,itertools,string,queue,datetime
sys.setrecursionlimit(10**8)
INF = float('inf')
mod = 10**9+7
eps = 10**-7
def inpl(): return list(map(int, input().split()))
def inpls(): return list(input().split())

a,b,c = inpl()
a *= 60
c *= 60*60

def check(N):
    t = a*N
    p = c+b*N
    return t >= p

if b >= a:
    print(-1)
else:
    OK = 10**30
    NG = 0
    while OK - NG > 1:
        mid = (OK+NG)//2
        if check(mid):
            OK = mid
        else:
            NG = mid
    print(OK)