import sys
sys.setrecursionlimit(10**9)
readline = sys.stdin.readline

def dfs(x, G, visited, log):
    visited[x] = True
    for y in G[x]:
        if visited[y]:
            continue
        dfs(y, G, visited, log)
    log.append(x)

def main():
    N = int(readline())
    fr = [[] for _ in range(N)]
    to = [[] for _ in range(N)]
    difficulty = [0]*N
    for x in range(N):
        d, y = map(int, readline().split())
        y -= 1
        fr[x].append(y)
        to[y].append(x)
        difficulty[x] = d

    visited = [False]*N
    log = []
    for x in range(N):
        if visited[x]:
            continue
        dfs(x, to, visited, log)
    log.reverse()

    visited = [False]*N
    groups = []
    for x in log:
        if visited[x]:
            continue
        group = []
        dfs(x, fr, visited, group)
        groups.append(group)

    I = len(groups)
    V_to_I = [-1]*N
    diff_of_I = [-1]*I
    for i, group in enumerate(groups):
        min_diff = 1000
        sum_diff = 0
        for x in group:
            V_to_I[x] = i
            min_diff = min(min_diff, difficulty[x])
            sum_diff += difficulty[x]
        diff_of_I[i] = (min_diff + sum_diff) / 2

    DAG = [[] for _ in range(I)]
    for x in range(N):
        i = V_to_I[x]
        for y in to[x]:
            j = V_to_I[y]
            if i != j:
                DAG[i].append(j)

    ans = 0
    visited = [False]*I
    for i in range(I):
        if visited[i]:
            continue
        component = []
        dfs(i, DAG, visited, component)
        component.reverse()
        for i, c in enumerate(component):
            if i == 0:
                ans += diff_of_I[c]
            else:
                ans += diff_of_I[c] / 2
    print(ans)

if __name__ == "__main__":
    main()