Grading Point in C

Question

This is my code Everything Works fine but when i put any alphabet not numbers it shows A-. i just want to know why this happening.Why it Shown only A-.Can anyone explain Why this happing?

#include<stdio.h>
int main(){
int num;
printf("Enter Your number ");
scanf("%d" ,& num);
    if(num>79)
    {
   printf("Congratulation you got A+");
}

else if(num>69)
{
printf("Congratulation you got A ");
}
else if(num>59)
{
printf("Congratulation you got A-");
}
else if(num>49)
    {
    printf("Congratulation you got B");
}
else if(num>39)
{
    printf("Congratulation you got C");
}

else if(num>32)
    {
    printf("Congratulation you got D");
}
else
{
    printf("You are a student like me(Shuvo) \nBetter Luck next time");

}
return 0 ;
}