import sys sys.setrecursionlimit(200005) int1 = lambda x: int(x)-1 p2D = lambda x: print(*x, sep="\n", end="\n\n") def II(): return int(sys.stdin.readline()) def LI(): return list(map(int, sys.stdin.readline().split())) def LLI(rows_number): return [LI() for _ in range(rows_number)] def LI1(): return list(map(int1, sys.stdin.readline().split())) def LLI1(rows_number): return [LI1() for _ in range(rows_number)] def SI(): return sys.stdin.readline().rstrip() dij = [(0, 1), (1, 0), (0, -1), (-1, 0)] # dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)] # inf = 18446744073709551615 inf = 4294967295 md = 10**9+7 # md = 998244353 # ax-bx >= c # x >= c/(a-b) a, b, c = LI() a *= 60 c *= 3600 if a <= b: print(-1) exit(0) print((c+a-b-1)//(a-b))