C++ :

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
#define MAXN 1000000+5
long A[MAXN],B[MAXN],tmp[MAXN];
long n,L,R;
long long P,Q,Ans,ans;
void read(long &x){
    char ch;
    for(ch=getchar();ch<'0'||ch>'9';ch=getchar());
    x=ch-'0';
    for(ch=getchar();ch>='0'&&ch<='9';ch=getchar())x=10*x+ch-'0';
}
long long gcd(long long a,long long b){
    return(!(a%b))?(b):(gcd(b,a%b));
}
void count1(long *a,long *b,long l,long r){
    if(l==r)return;
    long mid=(l+r)>>1;
    count1(a,b,l,mid);
    count1(a,b,mid+1,r);
    long i=l,j=mid+1,k=l-1;
    while(i<=mid && j<=r){
        while(a[i]<a[j] && i<=mid){
            b[++k]=a[i];
            if(j<=r)ans+=j-mid-1;
            i++;
        }
        while(a[i]>=a[j] && j<=r){
            b[++k]=a[j];
            j++;
        }
    }
    while(i<=mid){
        b[++k]=a[i];
        ans+=j-mid-1;
        i++;
    }
    for(long i=l;i<=j-1;i++)a[i]=b[i];
}
void count2(long *a,long *b,long l,long r){
    if(l==r)return;
    long mid=(l+r)>>1;
    count2(a,b,l,mid);
    count2(a,b,mid+1,r);
    long i=l,j=mid+1,k=l-1;
    while(i<=mid && j<=r){
        while(a[i]<=a[j] && i<=mid){
            b[++k]=a[i];
            if(j<=r)ans+=j-mid-1;
            i++;
        }
        while(a[i]>a[j] && j<=r){
            b[++k]=a[j];
            j++;
        }
    }
    while(i<=mid){
        b[++k]=a[i];
        ans+=j-mid-1;
        i++;
    }
    for(long i=l;i<=j-1;i++)a[i]=b[i];
}
int main()
{
	//freopen("game.in","r",stdin);
	//freopen("game.out","w",stdout);
    read(n);read(L);read(R);
    long sum=0;
    long t;
    A[0]=B[0]=0;
    for(long i=1;i<=n;i++){
        read(t);
        sum+=t;
        A[i]=L*i-sum;
        B[i]=R*i-sum;
    }
    ans=0;
    count1(A,tmp,0,n);
    Ans=ans;
    ans=0;
    count2(B,tmp,0,n);
    Ans-=ans;
    P=Ans;
    Q=(n*1LL/2)*(n*1LL+1LL);
    long long m=gcd(P,Q);
    cout<<P/m<<"/"<<Q/m<<endl;
    return 0;
}