POJ2096 Collecting Bugs(期望dp)

#include<cstdio>
#include<cstring>
using namespace std;
const int MAXN = 1051;
int N, S;
double f[MAXN][MAXN];
int main() {
    scanf("%d %d", &N, &S);
        f[N][S] = 0;
        for(int i = N; i >= 0; i--)
            for(int j = S; j >= 0; j--)
                if((i != N) || (j != S))
                    f[i][j] = (f[i + 1][j + 1] * (N - i) * (S - j) + f[i + 1][j] * (N - i) * j + f[i][j + 1] * i * (S - j) + N * S) / (N * S - i * j);
        printf("%.4lfn", f[0][0]);        
    return 0;
}