import sys

input = sys.stdin.readline


def main():
    N = int(input())
    L = []
    S = []
    for i in range(N):
        l, s = map(int, input().split())
        L.append(l)
        S.append(s)

    X = set()

    for i in range(N):
        for _ in range(N):
            i = S[i] - 1
        a = i
        for _ in range(N):
            i = S[i] - 1
            a = min(a, i, key=lambda x: (L[x], x))
        X.add(a)

    ans = 0
    for i in range(N):
        if i in X:
            ans += L[i]
        else:
            ans += L[i] / 2

    print(f'{ans:.1f}')


if __name__ == '__main__':
    main()