为什么这样不能通过

int main(){
    char a[1005];
    char b[1005];
    scanf("%s,%s",a,b);
    int i = 0;
    int sum = 0;
    while(i<sizeof(a)-1){
        if(a[i]!=b[i]){
        	a[i+1]=(a[i+1]=='*'?'o':'*');
            sum++;
        }
        i++;
    }
    printf("%d",sum);
    return 0;
}