let n = readLine()!.split(separator: " ").map{Int($0)!}
let (a,b,c) = (n[0] * 60,n[1],n[2] * 3600)
var x = 1
if a > b{
    while c+b*x >= a*x{
        x += 1
    }
}else{
    x = -1
}
print(x)