invalid conversion from char*' to char'

#include <stdio.h>
#include <string.h>
int f(char a[10], char b[10]) {
	int c,d;
	for (c=0;a[c]!=0;c++) ;
	for (d=0;b[d]!=0;d++) ;
	if (c!=d) return 1;
	else {
		int e=0;
		while (a[e]==b[e]) e++;
		if (e==c) return 2;
		else {
			for (e=0;a[e]>='a'&& a[e]<='z';) a[e]=a[e]-'a'+'A';
			for (e=0;b[e]>='a'&& b[e]<='z';) b[e]=b[e]-'a'+'A';
			e=0;
			while (a[e]==b[e]) e++;
			if (e==c) return 3;
			else return 4;
		} 
	}
}
int main () {
	char a[10]="0",b[10]="0";
	scanf ("%s",a);
	scanf ("%s",b);
	printf ("%d",f(a[10],b[10]));
}