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long long bs(long long key){
    long long left=0,right=n-1,mid;
    while(left<=right){
        int mid=(left+right)/2;
        if(a[mid]==key)
            return mid;
        else if(a[mid]>key)
            right=mid-1;
        else
            left=mid+1;                  
    }
    return -1;    
}

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