import math

def correlation(list1, list2):
    n = min(len(list1), len(list2))
    if n < 2:
        return None
    
    x, y = list1[:n], list2[:n]
    mean_x, mean_y = sum(x)/n, sum(y)/n
    
    num = sum((x[i]-mean_x)*(y[i]-mean_y) for i in range(n))
    den = math.sqrt(sum((x[i]-mean_x)**2 for i in range(n)) * 
                    sum((y[i]-mean_y)**2 for i in range(n)))
    
    return num/den if den != 0 else None