[ans_{i}=frac{sum_{i=l}^r dbinom{2}{cnt_{i}}}{dbinom{2}{r-l+1}}
]
[ans_{i}=frac{sum_{i=l}^r frac{cnt_{i} imes (cnt_{i}-1)}{2}}{frac{(r-l+1) imes (r-l)}{2}}
]
[ans_{i}=frac{sum_{i=l}^r cnt_{i} imes (cnt_{i}-1)}{(r-l+1) imes (r-l)}
]
[ans_{i}=frac{sum_{i=l}^r cnt_{i}^2-cnt_{i}}{(r-l+1) imes (r-l)}
]
[ans_{i}=frac{sum_{i=l}^r cnt_{i}^2 - sum_{i=l}^r cnt_{i}}{(r-l+1) imes (r-l)}
]
因为
[sum_{i=l}^r cnt_{i}=r-l+1
]
所以
[ans_{i}=frac{sum_{i=l}^r cnt_{i}^2 -(r-l+1)}{(r-l+1) imes (r-l)}
]