1 /************************************************************** 2     Problem: 1224 3     User: rausen 4     Language: C++ 5     Result: Accepted 6     Time:6800 ms 7     Memory:804 kb 8 ****************************************************************/ 9  10 #include <cstdio>11 #include <cmath>12  13 using namespace std;14 typedef double lf;15 const lf eps = (lf) 1e-10;16 const int N = 60;17  18 int n, m, ans;19 lf x, y, T;20 lf s[N];21  22 void dfs(int pos, int cnt, lf now) {23     lf Max = now + s[pos + (n - cnt) - 1] - s[pos - 1],24     Min = now + s[m] - s[m - (n - cnt)];25     if (Min - T > eps || Max - T < -eps) return;26     if (cnt == n) {27         ++ans;28         return;29     }30     if (pos == m + 1) return;31     dfs(pos + 1, cnt, now);32     dfs(pos + 1, cnt + 1, now + (lf) 1 / pos);33 }34  35 int main() {36     int i;37     scanf("%d%d%lf%lf", &n, &m, &x, &y);38     T = (lf) x / y;39     for (i = 1; i <= m; ++i)40         s[i] = s[i - 1] + (lf) 1 / i;41     dfs(1, 0, 0);42     printf("%d\n", ans);43     return 0;44 }