C :

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

void main(){
	char a[81];
	int i;
	gets(a);
	
	for(i=0;i<strlen(a);i++){
		if(a[i]>='A'&&a[i]<='Z'){
			a[i]=a[i]+32;
		}else if(a[i]>='a'&&a[i]<='z'){
			a[i]=a[i]-32;
		}
	}
	printf("%s",a);
}

Python :

s=input()
s1=""
for i in s:
    if i>="A" and i<="Z":
        s1=s1+i.lower()
    elif i>="a" and i<="z":
        s1=s1+i.upper()
    else:
        s1=s1+i
print(s1)