Program to input 2 numbers as strings and find the sum2 - C Code Bank

                #include<string.h>
#include<stdio.h>


int convert(char *num)
{
int l,power,n=0,neg=0;
l=strlen(num);
power=1;
for(int i=0;i<l-1;i++)
{
 power*=10;
 }

for(i=0;i<l;i++)
{
 switch(num[i])
  {
   case '-':if(num[0]=='-')
	 neg=1;
	 else
	{ printf("\nNOT A NUMBER");return(NULL);   }
	 break;
   case '1':n+=(1*power);break;
   case '2':n+=(2*power);break;
   case '3':n+=(3*power);break;
   case '4':n+=(4*power);break;
   case '5':n+=(5*power);break;
   case '6':n+=(6*power);break;
   case '7':n+=(7*power);break;
   case '8':n+=(8*power);break;
   case '9':n+=(9*power);break;
   case '0':n+=(0*power);break;
   default:cout<<"NOT A NUMBER";return(NULL);
 }
power/=10;

if(power==0)
 power=1;

}

if(neg==1)
 n*=-1;

return n;
}

void main()
{
char *num1,*num2;
int a,b,s;
clrscr();
printf("\nA program that converts string in to numbers using a 

function and adds 2 numbers\n\n");
printf("\nEnter 2 numbers: ");
scanf("%s",num1);
scanf("%s",num2);
a=convert(num1);
b=convert(num2);
if(a!=NULL || b!=NULL)
{s=a+b;
printf("\na=%d   b=%d   and sum=%d",a,b,sum);
}
else
printf("\nNOT A NUMBER");

}