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Cod sursa(job #700515)
#include<cstdio>
#include<vector>
#include<algorithm>
using namespace std;
int main()
{
vector<float> a;
int i,j,n,m,k,p=0;
float x,s=0;
freopen("dezastru.in","r",stdin);
freopen("dezastru.out","w",stdout);
scanf("%d%d",&n,&k);
for(i=1;i<=n;i++)
{
scanf("%f",&x);
a.push_back(x);
}
sort(a.begin(),a.end());
do
{
x=1; p++;
for(i=0;i<k;i++)
{
x*=a[i];
}
s+=x;
//printf("%f %f\n",x,s);
}
while(next_permutation(a.begin(),a.end()));
s=s/p;
printf("%f",s);
return 0;
}