题目链接

既然逆序 所以是N - 1的状态

//MADE BY Y_is_sunshine;
//#include <bits/stdc++.h>
//#include <memory.h>
#include <algorithm>
#include <iostream>
#include <cstdlib>
#include <cstring>
#include <sstream>
#include <cstdio>
#include <vector>
#include <string>
#include <cmath>
#include <queue>
#include <stack>
#include <map>
#include <set>

#define INF 0x3f3f3f3f
#define MAXN 1005

const int mod = 1e9 + 7;
const double PI = acos(-1);

using namespace std;


int fly[100010];
double dp[100010];

int main()
{
	freopen("data.txt", "r", stdin);

	int N, M;
	while (cin >> N >> M) {
		if (!(N | M))
			break;
		memset(dp, 0, sizeof(dp));
		memset(fly, -1, sizeof(fly));
		while (M--) {
			int a, b;
			cin >> a >> b;
			fly[a] = b;
		}
		
		for (int i = N - 1; i >= 0; i--) {
			if (fly[i] != -1)
				dp[i] += dp[fly[i]];
			else {
				for (int j = 1; j < 7; j++) {
					dp[i] += dp[i + j] * 1.0 / 6 ;
				}
				dp[i]++;
			}
		}
		printf("%.4lf\n", dp[0]);
	}


	freopen("CON", "r", stdin);
	system("pause");
	return 0;
}