class Problem0410:
    def solve(this):
        p1,p2 = map(int, input().split())
        q1,q2 = map(int, input().split())
        print(abs(p1 - q1) / 2 + abs(p2 - q2) / 2)

if __name__ == "__main__":
    problem = Problem0410()
    problem.solve()