func uniquePaths(m int, n int) int {
	f := make([]int, n+1)
	for i := range f {
		f[i] = 1
	}
	for i := 1; i < m; i++ {
		for j := 1; j < n; j++ {
			f[j] += f[j-1]
		}
	}
	return f[n-1]
}