import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

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

A *= 60
C *= 3600

if B >= A:
    print(-1)
    exit()

def test(x):
    # 書く
    return A*x >= C + B*x

left = 0
right = 10**20
while left + 1 < right:
    x = (left+right) // 2
    if test(x):
        right = x
    else:
        left = x

print(right)