Ubuntu bash shell scripting if/else statements

i'm doing a grade calculation for an assignment and i'm having problem finding what's wrong with this code:

#!/bin/bash

read -p "Enter assignment mark (0 to 40):" assignment
read -p "Enter Test1 mark (0 to 15):" testo
read -p "Enter Test2 mark (1 to 15):" testa
read -p "Enter Final exam mark (1 to 30" finalexam

grades = $(($assignment + $testo + $testa + $finalexam))
if [ $grades -ge 0 ] && [ $grades -lt 49 ] 
then
        echo "Your final grade is F"
elif [ $grades -ge 50 ] && [ $grades -lt 52 ]
then
        echo "Your final grade is D-"
elif [ $grades -ge 53 ] && [ $grades -lt 56 ]
then 
        echo "Your final grade is D"
elif [ $grades -ge 57 ] && [ $grades -lt 59 ]
then 
        echo "Your final grade is D+"
elif [ $grades -ge 60 ] && [ $grades -lt 62 ]
then 
        echo "Your final grade is C-"
elif [ $grades -ge 63 ] && [ $grades -lt 66 ]
then 
        echo "Your final grade is C"
elif [ $grades -ge 67 ] && [ $grades -lt 69 ]
then 
        echo "Your final grade is C+"
elif [ $grades -ge 70 ] && [ $grades -lt 72 ]
then 
        echo "Your final grade is B-"
elif [ $grades -ge 73 ] && [ $grades -lt 76 ]
then 
        echo "Your final grade is B"
elif [ $grades -ge 77 ] && [ $grades -lt 79 ]
then 
        echo "Your final grade is B+"
elif [ $grades -ge 80 ] && [ $grades -lt 84 ]
then 
        echo "Your final grade is A-"
elif [ $grades -ge 85 ] && [ $grades -lt 89 ]
then 
        echo "Your final grade is A"
elif [ $grades -ge 90 ] && [ $grades -lt 100 ]
then 
        echo "Your final grade is A+"
else
echo "Wrong input"
fi

It gives me an error when it comes to display the final grades. I've been trying for a long time but i still can't find it. Thank you for the help and for your time.