def main():
    P, p = map(int, input().split())
    Q, q = map(int, input().split())
    x = [P, Q]
    y = [p, q]
    X = (max(x)-min(x))/2
    Y = (max(y)-min(y))/2
    print(X+Y)
    
if __name__ == '__main__':
    main()